The Mexican Revolution involved a civil war with hundreds of thousands of deaths and large numbers fleeing combat zones. Tens of thousands fled to the U.S. President Wilson sent U.S. forces to occupy the Mexican city of Veracruz for six months in 1914. It was designed to show the U.S. was keenly interested in the civil war and would not tolerate attacks on Americans, especially the April 9, 1914, "Tampico Affair", which involved the arrest of American sailors by soldiers of the regime of Mexican President Victoriano Huerta. In early 1916 Pancho Villa a Mexican general ordered 500 soldiers on a murderous raid on the American city of Columbus New Mexico, with the goal of robbing banks to fund his army. The German Secret Service encouraged Pancho Villa in his attacks to involve the United States in an intervention in Mexico which would distract the United States from its growing involvement in the war and divert aid from Europe to support the intervention. Wilson called up the state militias (National Guard) and sent them and the U.S. Army under General John J. Pershing to punish Villa in the Pancho Villa Expedition. Villa fled, with the Americans in pursuit deep into Mexico, thereby arousing Mexican nationalism. By early 1917 President Venustiano Carranza had contained Villa and secured the border, so Wilson ordered Pershing to withdraw.

What early 20th century conflict sent tens of thousands of refugees to the US?