Final answer:
After the Civil War, the U.S. military built forts on the frontier to aid in westward expansion and support settlers during their migration. These forts served various purposes, including facilitating trade, protecting the coastline, and providing relief for farmers.
Step-by-step explanation:
After the Civil War, the U.S. military built forts on the frontier to aid in westward expansion. These forts were strategically located throughout the West to facilitate trade, limit conflict with Native American tribes, and provide relief for farmers during times of drought or hardships. They also played a vital role in the development of the region by serving as trading posts and supporting the growth of Pacific trade routes.