The first foreign country the U.S. stepped into a state of war with on the North American continent was Mexico in what is referred to as the Mexican-American War. The war began in 1846 and ended in 1848 with the Treaty of Hidalgo.

The Mexican-American War was sparked by the disputed territory of Texas. The U.S. was interested in the territory, but there was a conflict: Mexico laid claim to it, and additionally, the U.S. did not want to alienate the Mexican government by creating another slave state. 

President James K. Polk was a strong supporter of the idea of Manifest Destiny, which is the idea that the United States should own the land from the Atlantic to the Pacific. Polk had his eyes on the territories of Texas, New Mexico, and California. In 1845, troops were sent into Texas to the Nueces River, which Mexico still laid claim to. 

Once troops were stationed there, Mexican troops attacked and passed the Rio Grande. This was seen as an act of aggression to Polk, who then asked Congress to declare war on Mexico. The declaration of war was put into action two days later. 

The war lasted for two years. The U.S. and Mexico started peace negotiations after Mexico City surrendered to U.S. troops.

The war came to an official end on Feb. 2, 1848, when Mexico signed the Treaty of Hidalgo. This treaty would give the U.S. 545,000 square miles of territory.

These territories are what are known as present-day New Mexico, Arizona, Colorado, Utah, California, Nevada and a small part of Wyoming. 

Mexico was to relinquish its claim to Texas and also to recognize the Rio Grande as the official border between the two countries.

Although this may sound very costly for Mexico, the U.S. in return paid Mexico $15 million, which is equivalent to $487 million today. But that’s less than half of what Washington D.C. had originally offered.

The aftermath of this war affected both countries. Mexico suffered casualties and damages, its economy was in even more ruin, and of course they lost land.

There is some debate whether the end of this war would later lead to the American Civil War, due to the increase in land and a dispute as to whether the new states would be slave states or not.

Thirteen years later, the U.S. would be embroiled in the Civil War.