This article uses the Spanish-American War, one of the only possible cases of war between democracies, to demonstrate the power of a constructivist approach for explaining the liberal peace. A `structural idealist' argument suggests that the liberal peace must be conceived of as an `intersubjective consensus' of the leaders and citizens of liberal states. A Gramscian perspective interprets the liberal pacific union as an effort by the United States to exercise its global leadership through an ideological hegemony rather than through coercion. US perceptions that Spain was not a liberal democracy in 1898 and US efforts to legitimate its expansionism as necessary for the spread of democracy to the island of Cuba explain how two states that appeared to share liberal characteristics could nevertheless find themselves at war. In the aftermath of the war, the contrasting experiences of the USA in the Philippines and Cuba demonstrated the utility of pursuing a liberal policy for legitimating an expansionist policy at home and abroad. US efforts to impose colonial rule in the Philippines generated opposition in the United States and an insurgent movement in the Philippines. US efforts to craft a protectorate relationship with a Cuban government chosen in US-sponsored elections, however, met with widespread support in the USA and was accepted by Cuban nationalists.