The 2022 Russian invasion of Ukraine is consistent with the logic of preventive war. States often initiate wars because they fear the consequences of a shifting balance of military power and thus strike to forestall it. They fear that, once the balance changes, the rising power may either attempt to coerce them, or initiate war later under much more favorable circumstances. The tendency to consider preventive war is exacerbated if the declining state also sees itself as having a special, and fleeting, window of opportunity to prevent the shift. This essay reviews a range of evidence to argue that Vladimir Putin likely viewed Russia's strategic situation through a preventive war frame. NATO membership for Ukraine would shift the balance of power against Russia, and U.S. and NATO military cooperation with Ukraine intensified during the Joe Biden administration. These developments likely convinced Putin that he did not have much time to forestall Ukraine's NATO membership.