Tensions.

After 1845 the fact that Texas was now part of the United States was beyond dispute. Yet annexation did not calm tensions since the Texas-Mexican border remained a subject of dispute. The Mexican government defined the south and west border of Texas at the Nueces River. The Texas government, now backed by the U.S. government, declared that the Rio Grande was its southern border, a claim that increased the size of Texas by almost 200 percent. Predictably, Mexico responded to the Texans' belligerent declaration by breaking off diplomatic ties with the United States. This move set in motion a series of events that changed the pattern of Western expansion, foreign relations with Mexico, and American politics.

The Oregon Question.

In 1844 Democrat James K. Polk had run for president on a platform that called for the immediate annexation of...