Territorial expansion 240 History 182 Foreign relations 54 Imperialism 46 Politics and government 40 Indians of North America 39 Frontier and pioneer life 32 Commerce 18 Political aspects 18 Slavery 18 Government relations 16 Economic conditions 15 Race relations 15 Manifest Destiny 14 Racism 12 Power (Social sciences) 11 Social aspects 11 Causes 10 Discovery and exploration 10 African Americans 9 Extension to the territories 9 Political culture 9 Wars 9 Historiography 8 Louisiana Purchase 8 National characteristics, American 8 Nationalism 8 Public opinion 8 Spanish-American War, 1898 8 Civilization 7