During the 1840s and 1850s, the Corps was very active. John C. Frémont led several western explorations starting in 1841. He led an exploration of the North Platte River. He next explored west to what is now Pueblo, Colorado. His next exploration was south going through Bent's Fort on the Arkansas River. With Kit Carson as his guide, Fremont began a two year journey that greatly expanded the knowledge of what lay to the west. It took him as far as the Sacramento Valley in early 1846. They were also involved in surveying boundary lines, especially in Texas during the Mexican–American War. Until they were merged with the Corps of Engineers in 1863, they surveyed and built roads bridges and forts for the Union Army during the Civil War.