The Mexican American War took place between 1846 and 1848 between the United States and Mexico.

The Background
The conflict had its roots in the Texas War for Independence. Texas was part of Mexico, and decided they’d rather not be (good decision). They fought for independence, and ended up capturing the Mexican General in charge, Antonio López de Santa Anna.

Santa Anna signed a treaty setting the borders for Texas, and promising to recognize the independent republic. The Mexican government, however, refused to sign the treaty, saying Santa Anna signed it under duress.

Mexico always contended that Texas ended at the Nueces River, while Texas (and the US) contended that Texas ended at the Rio Grande.

The Start of War
Things really came to a head when the US annexed Texas. Mexico was opposed to the action, and the US annexed all of the land (to the Rio Grande), some of which Mexico felt was still theirs.

President James K. Polk sent an envoy to Mexico offering to purchase the disputed part of Texas, New Mexico, and California for $25 million. The Mexican government refused.

At the same time, Polk ordered General Zachary Taylor and his troops into the disputed part of Texas to defend the US land there. Mexico, still claiming the land was theirs, had their own troops there.

On April 25, 1846, 2,000 Mexican troops attacked a 63 man US patrol in the disputed area, killing most of them. Polk used this plus the rebuked offer of purchasing the land as the reasoning for obtaining a declaration of war against Mexico.

The War
I won’t go into the details of the war, but it lasted about 1 1/2 years, and obviously the US won, and Mexico lost. The war ended when US troops occupied Mexico City.

The Aftermath
Representatives of the US and Mexico signed the Treaty of Guadalupe Hidalgo in 1848. Mexico agreed to recognize the Texas border at the Rio Grande, and ceded a large portion of land in what is now the western US. It included most of what is now California, Arizona, Utah, Nevada, and New Mexico, and parts of Colorado and Wyoming.

Mexico got $18 million plus the US assumed $3 million in Mexican debt to US citizens.

Mexico lost about 55% of its land as a result of losing this war. Or 66% if you include Texas.

The Lesson
Don’t Mess With Texas.

Source: wiki and PBS