Anti-imperialism is a means of attacking concentrated power generally, breaking up empires, and attacking the largest states, which are usually the worst states in terms of their overall level of predation. The proliferation of more countries is a stepping stone toward abolishing states generally. However, is that sometimes imperial powers will try to weaponize regionalist movements against rival states, e.g. the US effort to co-opt the Hong Kong rebellion, the relationship between Catalonia or the Scottish National Party and the EU, or the affinity of Brexit’s leadership for Trumpism. It’s also possible to have more countries, but where these are submerged in an overarching transnational imperial system involving institutions like the UN, IMF, WTO, World Bank, multinational corporations, etc., just like it would possible to splinter California into multiple states but with all of these still subordinated to D.C. However, it’s also possible for regions, localities, and groups to achieve some degree of independence even within overarching systems, e.g. microstates like Lichenstein and Andorra, the millets in the Ottoman Empire, etc.
Anti-imperialism is a term used in a wide variety of contexts, usually by nationalist movements who want to secede from a larger body politic (often in the form of an empire, but also in a multi-ethnic sovereign state) or as a specific theory opposed to capitalism in Marxist–Leninist discourse, derived from Vladimir Lenin’s work Imperialism, the Highest Stage of Capitalism. People who categorize themselves as anti-imperialists often state that they are opposed to colonialism, colonial empires, hegemony, imperialism and the territorial expansion of countries.