1. President Polk claimed the Rio Grande boundary, and when Mexico sent forces over the Rio Grande, this provoked a dispute.
2. Most Texians wanted to join the United States of America, but annexation of Texas was contentious in the US Congress, where Whigs were largely opposed. Sixty-seven Whigs voted against the war on a key slavery amendment, but on the final passage only 14 Whigs voted no. They accused the President with usurping the war-making power.
3. In 1836 Mexico was relatively united in refusing to recognize the independence of Texas. Mexico threatened war with the United States if it annexed the Republic of Texas. Mexico laid claim to all the lands as far north as the Nueces River—about 150 mi (240 km) north of the Rio Grande.
4. The territorial expansion of the United States toward the Pacific coast had been the goal of US President James K. Polk
5. President Polk claimed the Rio Grande boundary, and when Mexico sent forces over the Rio Grande, this provoked a dispute.