Gradualism is common in international cooperation, as states begin with limited cooperation and choose more ambitious targets slowly over time. However, most models of international cooperation are static and thus cannot explain gradualism. I show that when states can implement domestic reforms to reduce the cost of international cooperation, enforcement concerns prompt gradualism. First, to achieve ambitious international cooperation, states must internationally enforce costly domestic reform. Second, defection at the reform stage is particularly pernicious. When other states reform, their ability to punish the defector by suspending cooperation is reduced. States choose the extent of reform to maximize the benefits of expected international cooperation in the future. As collective enforcement power increases or the number of states decreases, the extent and pace of reform increase. Gradualism is also most probable in international public good provision because excludable benefits mitigate enforcement problems.