Reconsidering the Benefits of Allies

What is the point of having allies, whether through a formal treaty or via informal security partnerships? The traditional understanding of alliances is that they are a means of achieving security via capability aggregation,Footnote ¹² or “external balancing.”Footnote ¹³ The work of Morrow helped scholars to broaden their understanding of the function of alliances to include nonsecurity benefits like enhancing the independent influence, or “autonomy,” of a major power in world politics.Footnote ¹⁴

Alliances and security partnerships can offer three sets of benefits: security, diplomatic, and economic. Such relationships can support economic ties, whether those allies provide raw materials,Footnote ¹⁵ offer trading markets,Footnote ¹⁶ or are willing to hold a security patron’s currency.Footnote ¹⁷ Diplomatic benefits pertain to the autonomy benefits highlighted by Morrow, the legitimacy conferred by even token participation by coalition partners in the military efforts of the major power,Footnote ¹⁸ and diplomatic support for the broader policy pursuits of a major power ally.Footnote ¹⁹ A benefit that sits at the intersection of diplomatic and security issues is greater control over the foreign and defense policies of allies, including decisions to initiate war, make defense investments, and develop operational plans. American security assurances can also reduce the desire of allies to weaponize their nuclear energy programs, as in South Korea and Japan.Footnote ²⁰

While economic and broader diplomatic benefits are important, our focus is on the third set of benefits: security. The security benefit most emphasized in past scholarship and by the Trump administration is the pooling of equipment and personnel to deter or fight a common adversary.Footnote ²¹ Exclusive focus on capability aggregation in the form of pooling resources can generate concerns about unequal burden sharing.Footnote ²² Scholars measure allies’ contribution to collective security via indicators of capability aggregation such as an ally’s level of defense spending,Footnote ²³ the complementarity of its defense investments,Footnote ²⁴ or its provision of combat power during war.Footnote ²⁵ These same allied investments in defense can help prevent wars from happening in the first place,Footnote ²⁶ although the efficacy of this deterrence benefit is difficult to empirically measure.Footnote ²⁷

While important, scholarly studies of allies largely overlook a second set of security benefits they offer: access. Allies, with or without formal treaties, can share the burden of security not only by spending on defense or contributing their troops and materiel to coalitions, but by allowing each other’s troops and materiel inside their borders. Access includes permission for military basing, which has been studied in existing work.Footnote ²⁸ But basing is just one dimension of access—and not necessarily the most important.

Allies grant access in many forms. Allies can allow states’ military services to transit through their territory, resupply in their ports, or fly through their skies. Allies can allow states’ intelligence services to build and operate surveillance infrastructure without ever committing to hosting a military base. Basing itself is a broad category of access that can be usefully disaggregated. An ally can let a foreign state own and operate bases from inside its sovereign territory; alternatively, the host can allow a state to use its own bases. An ally can allow a state to operate from its bases in peacetime only or to fight wars from inside its borders.

The value of access is rooted in a concept that has been underappreciated by scholars of alliances: the “tyranny of distance.” Kenneth Boulding introduced the concept of the “loss of strength” gradientFootnote ²⁹ in which the potency of military power diminishes the further away it is used. Boulding’s insight reflects the stubborn challenges raised by military logistics. Later scholarship has further shown how security-related missions and activities become more complicated at greater distance.Footnote ³⁰ A similar insight drives the logic of the “stopping power of water.”Footnote ³¹

Combat missions are harder to execute from a distance due to the practicalities that guns on tanks and aircraft do not shoot very far, aircraft need refueling, and short-range weapons systems must be transported close to the adversary. Sustainment likewise requires the transportation of the voluminous and bulky material necessary to keep the tip of the spear in the fight. Similarly, intelligence gathering is degraded by distance. Obtaining documents or insights from human sources requires intimate proximity. Wireless radio signals weaken as they travel. Reconnaissance aircraft require maintenance and refueling. There are exceptions—that is, intelligence collected at a distance via space-based platforms or cyber intrusions—but, on the whole, good intelligence is harder to obtain without close access.

Access offers a solution to tyranny of distance. If a major power can secure permission to use the territory, airspace, and waters of a foreign country that are near an adversary it wishes to deter, a theater of military operations, or a target of intelligence collection, this radically reduces the logistical burden of military and intelligence activity. Such access can, in turn, greatly increase the military and intelligence capability a major power can bring to bear against distant adversaries.

The Importance of Ally-Enabled Access

We now turn to explaining precisely how access has facilitated the projection of US military and intelligence power. We argue that access granted by the United States’ network of alliances and security partnerships has underpinned the US-led global system since 1945. Access has allowed Washington to deploy personnel and infrastructure near potential combat theaters, move materiel, conduct logistics, and fight directly from within foreign territory. Foreign partners have also permitted the United States to engage in intelligence collection activities. This has extended the reach of Washington’s ability to “see” and “hear.” In short, access has allowed the United States to overcome the tyranny of distance, command the commons,Footnote ³² and project power around the world.

The significance of access contributions for the projection of US military power can be understood across two scenarios: routine access during times of peace, and contingency access during times of war. Routine access during times of peace is perhaps the better understood and studied form of access. Allies permitting routine access, or forward presence, allow the US military to put its people and/or the infrastructure to support them inside their borders on a regular basis. The central purpose of forward presence facilitated by routine access is deterrence of interstate aggression, by punishment and by denial. The logic of “tripwire forces” holds that by putting US skin in the game, adversaries will calculate that attacking the ally will surely trigger a broader US intervention in the war. When US forward presence is more substantial, it can shift the local balance of military power such that adversaries calculate that they would be unable to achieve their military objectives quickly at reasonable cost.Footnote ³³

But routine access may not be the most important form of access offered by US allies. Allies often grant the US contingency access during times of war, meaning they allow the United States to conduct military operations against an adversary from inside their sovereign boundaries even if they are not under direct threat by an adversary. For every military operation, US allies not only decide whether, how many, and what kind of forces to contribute to the war effort, but also whether, how much, and what kind of contingency access to grant the US military. States can deny contingency access altogether, they can grant restricted contingency access, or they can grant total contingency access. States that grant total contingency access permit the US military to transit through their territory, skies, and/or waters en route to a target, conduct logistical operations in support of the military operation within their borders, and conduct direct combat operations from within their borders.

Consider Operation Desert Storm in 1991, hailed as the pinnacle demonstration of US military power. During Operation Desert Storm, the United States conducted a massive sealift and airlift to bases in Saudi Arabia, Oman, Qatar, the United Arab Emirates, and Turkey, stopping along the way at bases in Europe and North Africa. Egypt permitted the passage of US aircraft carriers through the Suez Canal. Once in theater, the United States Air Force decimated the Iraqi Army from runways in Oman, Qatar, the United Arab Emirates, Turkey and, of course, Saudi Arabia. In Desert Storm, the United States’ allies, both formal alliance members and security partners, contributed less than 15 percent of the coalition’s total sorties against the Iraqi Air Force and Iraqi Army. However, the vast majority of the airpower the United States brought to bear in the Gulf War was facilitated by access from the United States’ allies.Footnote ³⁴

Consider also NATO’s effort to stop President Slobodan Milosevic’s ethnic cleansing of Albanians. In the aerial bombing campaign Operation Allied Force, the coalition in total flew almost 40,000 sorties. While the United States flew the vast majority of the sorties and coalition partners flew only a small fraction, the United States flew almost entirely from bases inside coalition partners’ borders.Footnote ³⁵ Force contributions by US allies in Operation Allied Force might have been less than impressive, but their contributions in access were decisive.

More generally, try to imagine any US-led military operation since World War II without access: Vietnam. Iraq I. The wars in Iraq and Afghanistan. The ongoing war against the Islamic State. The United States generated the most combat power by far in each of these wars. But it did so largely from inside the borders of allies proximate to the targets. Without contingency access from its allies, the United States would have had to generate all its military power through naval aviation or long-range weapons systems. This would have dramatically raised the costs and lowered the effectiveness of the operations. Probably, for better or for worse, it would mean that the United States would not have conducted many of those operations at all because it could not have done so at an acceptable cost. It is unclear whether the United States would (or should) defend Taiwan if the People’s Republic of China attempts to seize it by force. It is clear, however, that without access from allies in the region, the United States would be unable to do so.Footnote ³⁶

In addition to facilitating the projection of US military power, access-granting allies facilitate the projection of US intelligence power. Intelligence, or the raw data and insights states obtain through clandestine means, enables diplomacy and military planning, for defensive missions (for example, warning of military attack) and offensive actions (for example, identifying targets for a bombing campaign). The collection of intelligence—meaning the methods states use to acquire raw informational data about foreign intentions, capabilities, and events—relies on its own kind of infrastructure. This includes the physical sites, installations, and buildings that house intelligence collection and analytic functions. Such sites need to be near the intelligence targets, which are not often physically close to the United States. For this reason, since 1945, the United States has consistently and broadly relied on allies to permit building installations and operate intelligence collection activities on its territory.

The infrastructure for collection activity is easy to overlook but an essential part of its logistics. Overhead imagery obtained via drones or crewed aircraft rely on foreign airfields for take-off and landing and communication links to transmit photographs.Footnote ³⁷ Signals interception relies on some kind of sensor (that is, an antenna) positioned to detect a wireless radio or other signal at the correct frequency and direction.Footnote ³⁸ Seismic monitors in foreign locations enable states to monitor nuclear programs of other states.Footnote ³⁹ Human-source intelligence often requires intimate access inside the target of interest. Such logistics are part of the “how” of intelligence collection, the “sources and methods” that states work hard to hide.Footnote ⁴⁰

Declassification of US official records is shedding more light on intelligence collection logistics and the special role of its alliances and security partnerships. One example is Turkey, America’s NATO ally. Beginning in the mid-1950s, the United States operated a network of installations for intercepting missile telemetry signals and military communications along the Turkish coast of the Black Sea.Footnote ⁴¹ Access to Turkish territory allowed the United States the rare opportunity to build infrastructure close to Soviet missile testing grounds and military/naval movements in the Black Sea. As one history notes, “no piece of real estate during the Cold War bulked larger than Turkey” from foreign surveillance activity and this activity “came to dominate American–Turkish relations.”Footnote ⁴²

Another example, also within the NATO alliance, is Denmark. Denmark’s control of Greenland offered the United States a strategically valuable position in the Far North that was highly valuable for American intelligence collection and early warning. The shortest route for any attack by Moscow was over the North Pole. Early warning installations built in Greenland could detect incoming Soviet bombers and, later, ballistic missiles.Footnote ⁴³ Decades later, the American presence in Greenland has evolved into an important installation for space situational awareness and, in the second Trump administration, a potential new patch of American territory.Footnote ⁴⁴

The story in East Asia was similar. Along with controversial military bases in places like Okinawa, the United States also built and operated sensitive intelligence collection installations in Japan. A declassified National Security Agency (NSA) history notes the important role of signal intercept sites at Misawa and Wakkanai in Japan, which allowed the United States to listen in on Soviet military chatter during its 1983 shootdown of civilian flight KAL 007.Footnote ⁴⁵ During the Vietnam War, Washington relied on access to Thailand for its surveillance activities. The NSA “used Thailand as a principal base of cryptologic operations”Footnote ⁴⁶ while the CIA flew flights from Thai territory.Footnote ⁴⁷

Even as surveillance technology evolved, US allies continued to provide important forms of access. In the 1960s, the United States obtained access to British-controlled Seychelles islands in the Indian Ocean to build a space tracking installation for its new satellite reconnaissance program.Footnote ⁴⁸ Later the United States built installations in Australia. Later exposed by Edward Snowden,Footnote ⁴⁹ a large installation at Pine Gap has, for decades, enabled American satellite-based detection of Soviet missile launches and interception of signals.Footnote ⁵⁰ Before its revolution in 1979, American leaders built a close relationship with the Shah in Iran in large part due to his willingness to permit the operation of sensitive intelligence facilities on Iranian territory.Footnote ⁵¹

Implications for the Future of US Foreign Policy and World Order

Given the benefits provided by of alliances and security partnerships, as well as the broader role they have played for the United States, what are their implications for the United States and the international order going forward? If the United States dramatically reorients its policies toward its allies, it will have significant implications for the diplomatic, economic, and security benefits it has long derived from those relationships. That, in turn, could reshape the world order.

Such questions arise now because of four broad patterns in the Trump administration’s approach to allies thus far. First, the United States has imposed historically high tariffs on its closest allies while demanding significant improvements in how US exports are treated. Second, it has aggressively demanded that US allies pay more for their defense. At the same time, Washington is using language and adopting policies that raise doubts about its own commitment to defending its allies and upholding broader norms of territorial integrity. Third, the Trump administration has by and large been toughest on its traditional liberal democratic allies while exhibiting a proclivity to take a softer line on autocratic adversaries of the United States. Fourth, the administration has engaged in broad attack on the institutions and practices of multilateralism and has embraced unilateral military action, most notably in the American military strike against Iran’s nuclear facilities and on a Venezuelan speedboat.

Returning to the prior typology, such actions could endanger the diplomatic, economic, and/or security benefits that alliances and security partnerships have historically provided. For example, allies may balk at offering diplomatic cover to an American administration that wavers in its commitments. A future effort to assemble a “coalition of the willing” for a military campaign could find fewer friends and allies willing to actively participate or otherwise lend public support. The economic implications of the Trump administration’s actions are likely to be mixed and will depend on the outcome of ongoing bargaining. On one hand, economic pressure via tariffs could yield improved investment commitments and terms of trade from allies. A US–Japan trade deal reached in September 2025 is one example. On the other hand, the shakier US security commitment to allies could make many of them less willing to offer favorable economic treatment. This and other stumbling blocks could prevent the United States from reaching generous new deals, leaving aggregate tariff levels higher and diminishing the economic perks of alliances.

While the implications for the diplomatic and economic benefits are important, perhaps the most consequential changes will be in the category of security. On the one hand, the Trump administration’s singular focus on the defense spending of allies has already produced meaningful increases in defense spending by its European allies, as in NATO’s commitment to invest 5 percent of GDP on defense by 2035. This could reduce the free riding that has long been a complaint of the US toward its allies.Footnote ⁵² Moral hazard should also be reduced, as allies less certain of the United States’ security umbrella should be more cautious about escalating conflicts against adversaries. Yet, over time, increased security independence does have a downside. The more capable the United States’ allies become, the more they can operate without US support and initiate policies—including starting crises or even wars—that the United States opposes. Nuclear proliferation is also more likely in a world characterized by wavering US assurances and more frequent interstate wars.

When it comes to access, the United States may lose the permissions that have facilitated US military and intelligence power projection since World War II. To be clear, some host states have a strong self-interest in US access. Countries like South Korea and Taiwan face direct threats from large security rivals; access could be critical to their survival. Yet in many other places, the United States will no longer be able to take for granted the access it may need. It has lost access in the past. Consider Turkey’s refusal to allow the US Army to open a northern front against Iraq in 2003. Or Spain and France’s refusal to allow the US Air Force to overfly their territory en route to bomb Libya in 1986. The United States lost access—sometimes overnight—to intelligence installations in Pakistan, Turkey, and Iran during the 1960s and 1970s. If the PRC attacks Taiwan, US military access to bases in South Korea and the Philippines are far from assured. Such instances would likely multiply.

States have many reasons to restrict or deny access, and the pressure and disruptions from Washington, combined with the shifting balance of power, may increase the perceived costs and reduce the perceived benefits of letting the United States in. Granting access increases the risk a host is the target of military attacks or sabotage. Allowing a foreign power like the United States to attack a neighbor from its territory can invite retaliation. Allowing the United States to collect intelligence can similarly attract the coercive attention of an adversary, a factor that led both Turkish and Australian leaders to press for reduced US access in the Cold War. Given the Trump administration’s wavering commitment to collective defense, such criticism could convince allies that they have little to gain from offering access to a state unwilling to protect them in return. This could be particularly decisive in a strategic environment characterized by rising powers and the proliferation of offensive weapons that make states increasingly vulnerable to attack.Footnote ⁵³

Allies may also restrict or deny access for reputational reasons. Hosts may deny access to spare themselves the reputational costs abroad and at home of abetting a great power whose foreign policy is increasingly viewed as illegitimate, particularly if the military operation for which access is requested is controversial. States may also deny the US access due to its support for allies engaged in what many foreign capitals see as human rights abuses, such as Saudi Arabia in the War in Yemen or Israel in the War in Gaza. The United States may find itself relying on hefty cash payments to try to buy access, and it might find itself unable to purchase the alliance benefits it previously enjoyed for a much lower cost. Future research can deepen and extend these ideas, including by analyzing the causes of variation in access, why access changes over time, and generating new empirical evidence on the consequences of different access arrangements.

In short, the current administration’s shift in approach toward its alliances and security partnerships may reduce some alliance costs but do significant damage to diplomatic and access-related security benefits. Loss of access, a form of burden sharing often taken for granted by policymakers, is a significant but overlooked risk that affects both American military and intelligence power. There is debate in International Relations scholarship and within the current administration about whether the United States should continue to project power globally or give up its global role and focus on its borders. Ending access would end the debate. Access denied would mean the end of American hegemony and the end of US-led world order.