USA HISTORY

Detailed explanation-1: -Mexico did not recognize Texas as legitimate American territory and Texas admission to the United States antagonized Mexican officials and citizens. Rather than ameliorate this problem, President Polk intentionally worked to aggravate Mexico and provoke a war.

Detailed explanation-2: -Texas claimed the Rio Grande as its southern border. Mexico said the Nueces River, to the north, should be the border.

Detailed explanation-3: -Mexico claimed that the southern boundary of Texas was the Nueces River, the Texan boundary while under Mexican rule. Americans, as well as the incoming President, claimed that the boundary of Texas was the Rio Grande River.

Detailed explanation-4: -In 1845 the United States annexed Texas and subsequently engaged in a dispute with Mexico over the southern Texas-Mexico border. Texas claimed that its southwest boundary extended to the Rio Grande. Mexico claimed that the boundary was the Nueces River, which is 100 miles (160 kilometers) eastward.