The United States occupation of Veracruz, which began with the Battle of Veracruz, lasted for six months and was a response to the Tampico Affair of April 9, 1914. The incident came in the midst of poor diplomatic relations between Mexico and the United States, related to the ongoing Mexican Revolution.
After the Tampico Affair, where nine American sailors were arrested by the Mexican government for entering off-limit areas in Tampico, Tamaulipas,President Woodrow Wilson ordered the United States Navy to prepare for the occupation of the port of Veracruz. While waiting for authorization of Congress to carry out such action, Wilson was alerted of a delivery of weapons for Victoriano Huerta due to arrive to the port on April 21 on the German-registered cargo-steamer SS Ypiranga. As a result, Wilson issued an immediate order to seize the port's customs office and confiscate the weaponry. Huerta had taken power with the assistance of the American ambassador Henry Lane Wilson during a coup d'état in early 1913 known as La decena trágica. The Wilson administration's answer to this was to declare Huerta a usurper of the legitimate government, embargo arms shipments to Huerta, and support the Constitutional Army of Venustiano Carranza.
(... from Wikipedia on 2012-04-18 02:43:51)