Abstract: Because federalism grants partial autonomy to subunits of a nation, it has potentially broad implications for the prevailing system of international law, which is centered around the integrity of nation states. Military intervention in the internal affairs of a nation to protect human rights or combat terrorist activity might be regarded as more justifiable if the people being protected are members of a federalized subunit. Alternatively, foreign nations may feel more justified in establishing trade or cultural relations with a subunit of a nation, over objections by the nation’s government, if the subunit has federalized status. In other words, federalism can be viewed as modifying the principle of national sovereignty. This important possibility is largely unexplored in the international law literature. In recent years, a great deal of attention has been devoted to the implications of American federalism for the treaty powers of our national government and for the foreign relations powers of the state governments. While these questions refer to international law, they are basically domestic law inquiries about the relationship between state and national government. This article, in contrast, deals directly with federalism’s implications for relations between different nations. On the basis of a theory that treats federalism as a sub-optimal compromise for dealing with conflicts of political identity, the article concludes that federalism does not justify increased levels of intervention, nor does it permit foreign nations to establish relations with subunits in violation of national policy. But the article does recognize an important role for federalism in international relations. One nation should not intervene by force in another nation unless it has some reason to believe its intervention can be effective in solving the problem that provided the basis for the intervention. In a number of cases, federalism may be such a solution. In addition, while nations should not establish trade or cultural relations with subunits in violation of national policy, federalism creates a presumption that certain relations are acceptable to the national government, and thus requires the national government of a federal regime to be more explicit in prohibiting such relations than the government of a unitary regime would need to be.