 Complete the following phrase:  After Texass annexation, Mexico broke diplomatic relations with the United States. While the United States claimed Texass border stretched to the Rio Grande, Mexico claimed it was the Nueces River leaving the Rio Grande Valley under contested Texan sovereignty. While the former Republic of Texas could not enforce its border claims, the United States had the military strength and the political will to do so. President Polk ordered General Zachary Taylor south to the Rio Grande on January 13, 1846. A few months later Mexican troops routed an American cavalry patrol in the disputed area in the Thornton Affair starting the MexicanAmerican War. The first battles of the war were fought in Texas: the Siege of Fort Texas, Battle of Palo Alto and 