War between the United States and Mexico arose from disputes regarding their international boundary and U.S. attempts to expand its territory into Mexico. After Mexico rejected his offer to purchase its northern region, President James K. Polk sent the U.S. Army into the disputed borderland. There Mexican forces wounded American troops, provoking Congress to declare war. U.S. forces won a major battle at Buena Vista in northeastern Mexico and captured Mexico City in 1847. In the peace settlement, Mexico ceded its northern territory to the United States.