The Mexican-American War was an armed conflict between the United States and Mexico that began in 1846 and ended in 1848. American forces invaded various Mexican territories and conducted blockades resulting in Mexico accepting new borders and being forgiven of its debts to the United States.
What caused the Mexican-American War?
Prior to the beginning of the Mexican-American war there had been signs of unrest along the borders of the two countries, with both trying to expand their territory. Tensions began to escalate after Texas declared independence from Mexico in 1835. At this time Mexico warned the United States that if they were to take Texas it would be seen as a declaration of war. When Texas joined the U.S as a state in 1845, Mexico cut all diplomatic relations with the United States.
When Texas first became an independent territory a treaty was signed by both parties. In this treaty the borders of Texas were set out. However, Mexico didn’t take the claim of independence seriously and thought that it would be taken back at some point. When Texas became a state of the United States they claimed the borders of Texas up to the Rio Grande. Mexico did not accept these borders as valid and claimed the border only reached to the Nueces River. The President of the United States, President Polk, claimed Texan territory extended all the way to the Rio Grande. This led to a dispute with Mexico over the land between the Nueces River and the Rio Grande. America stationed troops along the Nueces River to protect Texas from a Mexican Invasion.
While this dispute was still in play the US President made various moves to secure Alta California, a Mexican territory, and encourage the citizens of the area to fight for their independence from Mexico as Texas had done. This was seen as a serious threat by the Mexican government. The President then sent a secret diplomat to Mexico City with an offer of $25 million ($632,500,000 today) for the border of Texas to Rio Grande and for California, but it was refused.
After the refusal of the offer, the United States sent troops into the disputed zone between the Nueces River and the Rio Grande in Texas and constructed a makeshift fort at the Rio Grande. Mexico saw this as an obviously hostile act and sent armed forces to attack a small group of American soldiers traveling through the area. After this attack The Us Congress approved the declaration of war on Mexico, which occurred on May 13, 1846.