Summary
Highlights
The USA began the 19th century as a small, isolationist nation focused on internal matters like westward expansion and the Civil War. Key territorial acquisitions included the Louisiana Purchase (1803), Florida (1819), annexation of Texas (1845), the Oregon Treaty (1846), land from the Mexican-American War (1846-1848), and the Alaska Purchase (1867), which marked the end of continental expansion. This expansion was driven by the belief in 'Manifest Destiny,' the idea that U.S. expansion across North America was inevitable and divinely ordained to spread liberty and American values. The Monroe Doctrine (1823) declared European interference in the Americas as an act of aggression. By 1890, the frontier was declared closed due to widespread settlement, encouraged by acts like the Donation Land Claim Act of 1850 and Homestead Acts of 1862. This closure shifted American focus to seeking opportunities abroad, influenced by historian Frederick Jackson Turner's views on westward expansion shaping American values.
In the late 19th century, the US experienced significant industrial growth fueled by natural resources, population growth from immigration, mass production advances, and railroad development. However, economic challenges such as banking instability (e.g., the Panic of 1893) and over-reliance on domestic markets highlighted the need for overseas trade. Protectionist tariffs in Europe made access to Asian markets crucial. Naval expansion, influenced by Alfred T. Mahan's 'The Influence of Sea Power Upon History,' became a priority, with Theodore Roosevelt building a strong navy to protect growing overseas interests.
The Spanish-American War was caused by US support for Cuban independence, sensationalist 'Yellow Press' journalism, and the explosion of the USS Maine. The US also had significant economic interests in Cuban sugar plantations and expansionist goals in the Caribbean and Pacific. The swift US victory, formalized by the Treaty of Paris (1898), ceded Puerto Rico, Guam, and the Philippines to the US, signaling its emergence as a global power. Cuba gained nominal independence but remained under US influence through the Platt Amendment.
Theodore Roosevelt strongly advocated for US involvement in global affairs and imperialism. He negotiated treaties like the Treaty of Portsmouth (1905) and expanded naval capabilities. A key achievement was the Panama Canal, completed by 1914, which enhanced US trade routes and Pacific influence. The Roosevelt Corollary, an addition to the Monroe Doctrine, asserted the right of US intervention in Latin America to maintain stability and protect economic interests, justified as necessary for regional order and security.
Initially, the USA maintained a policy of neutrality in World War I, benefiting economically from it. However, unrestricted German submarine warfare, including U-boat attacks on American ships, and the Zimmerman Telegram (1917), where Germany proposed an alliance with Mexico against the US, led to US entry. The US played a decisive role by providing fresh troops and resources to the Allied forces, strengthening its economy through wartime production and loans. President Woodrow Wilson's Fourteen Points outlined principles for post-war peace, including open diplomacy, free navigation, reduced trade barriers and military forces, self-determination, and the creation of a League of Nations. After the war, the USA emerged as a dominant financial and global power, shifting from isolationism to active international participation.