In 1913, he gave the Monroe doctrine a moral tone. According to his missionary diplomacy, the US had a moral responsibility to deny recognition to any Latin American gov't it viewed as oppressive, undemocratic, or hostile to US interest. His policy pressured nations in the Western Hemisphere to establish democratic gov'ts. Almost immediately, the Mexican revolution put his policy to the test. Porfiro Díaz was overthrown by Francisco Madero then General Victoriano Huerta took over. Wilson refused to recognize the gov't that Huerta formed, and he adopted a plan of watchful waiting, looking for an opportunity to act against Huerta. The opportunity came in April 1914 when an incident arose, and Wilson use for as an excuse to intervene in Mexico and order US marines to occupy Veracruz. This brought the US an Mexico close to a war. Wilson eventually withdrew the troops and formally recognized the carranza gov't. Mexican rebels Francisco (Pancho) Villa and Emiliano Zapata opposed this gov't. Despite Villa's talk of friendship, when Wilson recognized carranza's gov't, Villa threatened reprisals against the US. Carrranza invited American engineers to operate mines in northern Mexico, but they never made it because Villa's men took the Americans off the train and shot them. With the American public demanding revenge, President Wilson ordered Brigadiers General John J. Pershing and 15,000 soldiers into Mexico to capture villa dead or alive. This made Mexico angry, and Carranza demanded that Wilson withdraw US troops, but he refused. Wilson ordered Pershing to return home.