<P> Texas, which had been settled by colonies of Americans, fought a successful war of independence against Mexico in 1836 . Mexico refused to recognize the independence and warned that annexation to the United States meant war . Annexation came in 1845 and war in 1846 . The American military was easily triumphant . The result was the Mexican Cession of Santa Fe de Nuevo México and Alta California . About 60,000 Mexicans remained in the territories and became US citizens . France took advantage of the American Civil War (1861--65), using its army to take over Mexico . Due to defeats in Europe, France pulled out troops, leaving the Imperialists and Maximilian I of Mexico to face defeat from the Benito Juarez - led Republicans (backed by the US). </P> <P> The Anglo - Venezuelan boundary dispute of Guayana Esequiba in 1895 asserted for the first time a more outward - looking American foreign policy, particularly in the Americas, marking the United States as a world power . This was the earliest example of modern interventionism under the Monroe Doctrine, in which the USA exercised its claimed prerogatives in the Americas . By the late nineteenth century the rapid economic growth of the United States increasingly troubled Latin America . A Pan-American Union was created under American aegis, but it had little impact as did its successor the Organization of American States . </P> <P> As unrest in Cuba escalated in the 1890s, the United States demanded reforms that Spain was unable to accomplish . The result was the short Spanish--American War of 1898, in which United States acquired Puerto Rico and set up a protectorate over Cuba under the Platt Amendment rule passed as part of the 1901 Army Appropriations Bill . The building of the Panama Canal absorbed American attention from 1903 . The US facilitated a revolt that made Panama independent and set up the Panama Canal Zone as an American owned and operated district that was finally returned to Panama in 1979 . The Canal opened in 1914 and proved a major factor in world trade . The United States paid special attention to protection of the military approaches to the Panama Canal, including threats by Germany . Repeatedly it seized temporary control of several countries, especially Haiti and Nicaragua . </P> <P> The Mexican Revolution started in 1911; it alarmed American business interests that had invested in Mexican mines and railways . The United States involvement in the Mexican Revolution, include, among other violations of sovereignty, the ambassadorial backing of a coup and assassination of President Francisco I. Madero and the military occupation of Veracruz . Large numbers of Mexicans fled the war - torn revolution into the southwestern United States . Meanwhile, the United States increasingly replaced Britain as the major trade partner and financier throughout Latin America . The US adopted a "Good Neighbor Policy" in the 1930s, which meant friendly trade relations would continue regardless of political conditions or dictatorships . United States signed up the major countries as allies against Germany and Japan in World War II . However, Argentina refused to accede . The era of the Good Neighbor Policy ended with the ramp - up of the Cold War in 1945, as the United States felt there was a greater need to protect the western hemisphere from Soviet Union influence and a potential rise of communism . These changes conflicted with the Good Neighbor Policy's fundamental principle of non-intervention and led to a new wave of US involvement in Latin American affairs . </P>

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