Westward expansion led to rising tensions in Congress before the Civil War. Some considered expansion a defense against European claims on the continent, but it was also a subject of dispute between northern and southern factions about whether slavery would be allowed in new territories. In 1846, following a skirmish near the border of newly annexed Texas, President James K. Polk asked Congress to declare war against Mexico. 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. In December 1847 Representative Abraham Lincoln of Illinois, suspicious of Polk’s motives, called for an investigation.