The two crises in this chapter share three main characteristics. They involve territorial (border) conflict that relates to the independence of Ukraine (or, relatedly, the breakup of the Soviet Union), feature an East–West tension, and (as of this writing) do not escalate to a war among the major states. In 2014, after Ukraine attempted to move closer to Europe (i.e., it contemplated an EU agreement and the pro-Russian government fell), Putin annexed Crimea to secure the long-held naval base there. Although done forcefully, there were no military fatalities. In 2022, amidst a fear that Ukraine was again moving closer to Europe (i.e., it looked to be closer to joining NATO and its government became less pro-Russian), Russia invaded Ukraine. It failed to take Kyiv, even though it heavily bombed Ukraine. Russia then withdrew to the east, where a majority of Russian speakers had sought to separate from Ukraine. The United States and the European Union gave weapons and aid that expanded as the war continued. Deaths mounted on both sides. The Russians successfully created a land bridge from the Donbas to Crimea. After his election, Trump attempted to negotiate a settlement that would end the war.

This chapter summarizes the main lessons for diplomacy that we derive from our study. These eight lessons are: 1. A major factor separates the crises that escalate to war from those that do not; in the latter, a strong leader reins in any hard-liners who advocate going to war. 2. Individuals make a difference. 3. Contingency plays a more important role than system structure in determining whether or not a crisis escalates to war. 4. Someone must stand for peace. 5. The secret to preventing war structurally is to find a functional equivalent to war. 6. Norms and rules are important for avoiding war – and, therefore, maintaining peace. 7. War can be avoided; it is not inevitable. 8. The realist concepts of the national interest and balance of power do not always accurately describe the behavior of states.

This chapter synthesizes information about the key variables in our framework across all of the cases in our study. We begin with the cases that escalate to war. Here, the patterns among our variables suggest six prominent (nonexclusive) “paths to war”: the hard-liner path, the territorial path, the alliance path, the rivalry path, the bargaining failure path, and the democratic path. The cases that do not escalate to war reinforce the importance of these paths. Actors often avoid war by blocking one or more paths to war. Reining in hard-liners blocks the hard-liner path to war; alliances sometimes restrain actors, rather than emboldening them, thereby blocking the alliance path to war; and norms often provide a way for states to settle the disputed issue at hand. Finally, we discuss the relative importance of system structure. Prior to 1945, domestic politics – particularly, controlling hard-liners – was a more important factor than system structure in the decisions through which actors avoided war. After 1945, however, nuclear weapons reduce the probability of war among the superpowers, often by creating more accommodationists.