On the morning of April 21, 1914, warships of the United States Atlantic Fleet under the command of Rear Admiral Frank Friday Fletcher, began preparations for the seizure of the Veracruz waterfront. At 11:12 hrs, consul William Canada watched from the roof of the American Consulate as the first boatload of Marines left the auxiliary vessel USS Prairie. By 11:30, with whaleboats swung over the side, 502 U.S. Marines from the 2nd Advanced Base Regiment, 285 armed Navy sailors, known as "Bluejackets," from the battleship USS Florida and a provisional battalion composed of the Marine detachments from Florida and her sister ship USS Utah also began landing operations. As the landing party moved toward pier 4, Veracruz's main wharf, a large crowd of Mexican and American citizens gathered to watch the spectacle. The invaders encountered no resistance as they exited the whaleboats, formed ranks into a Marine and a seaman regiment, and began marching toward their objectives. This initial show of force was enough to prompt the retreat of the Mexican forces led by General Gustavo Maass. In the face of this,  Commodore Manuel Azueta encouraged cadets of the Veracruz Naval Academy to take up the defense of the port for themselves. Also, about 200 line soldiers of the Mexican Army remained behind to fight the invaders along with the citizens of Veracruz.. From what USS ships did American servicemen attack Veracruz?