We present a theory of war onset and war duration in which power is multidimensional and can evolve through conflict. The resources players can secure without fighting are determined by their political power, while the ability of appropriating resources with violence is due to their military power. When deciding whether to wage a war, players evaluate the consequences on the current allocation of resources as well as on the future distribution of military and political powers. We deliver three main results: a key driver of war is the mismatch between military and political power; dynamic incentives may amplify static incentives, leading forward-looking players to be more belligerent; and a war is more likely to last for longer if political power is initially more unbalanced than military power and the politically under-represented player is militarily advantaged. Our results are robust to allowing the peaceful allocation of resources to be a function of both political and military powers. Finally, we provide empirical correlations on interstate wars that are consistent with the theory.