Why do leaders initiate costly wars? Traditional explanations emphasize empire building or resource extraction. We advance a different view: war as a tool of nation-building. Using the Russian invasion of Ukraine, we show that President Putin’s decision is best understood as an effort to consolidate domestic power by appealing to ethnic Russians and strengthening national identity in the face of declining popularity. Ethnic Russians are systematically more likely to turn out and support Putin, and this effect is not more substantial in places with symbols of imperial history, such as statues of Lenin and WWII monuments. Instead, the mobilization response is concentrated among areas with a pre-existing ethnic Russian presence. These patterns remain robust to alternative strategies that account for electoral fraud. We situate these findings within the broader literature on the causes of conflict and the political economy of nation-building. The evidence is most consistent with war being used less to expand the empire and more to forge internal cohesion, rally support, and preserve regime survival.