 Complete the following phrase:  The United States positioned troops along the Rio Grande. On April 25, 1846, in an event known as the Thornton Affair, a large contingent of Mexican cavalry attacked an American patrol in the area between the Rio Grande and the Nueces, killing 16 Americans. On May 3, Mexican troops initiated the siege of Fort Texas, bombarding a makeshift American fort along the Rio Grande. On May 8, Zachary Taylor led 2,500 U.S. troops to relieve the fort. He was intercepted by Mexican troops, leading to the Battle of Palo Alto. Mexican troops retreated a short distance to regroup, and the following day the two sides fought fiercely in the 