Complex sentence: After the War of 1812, the United States took over this area and the fur trade, refusing to license Canadian traders, with whom the Americans had previously passed back and forth across the border.
Split into simpler sentences: After the War of 1812, the United States took over this area and the fur trade. They refused to license Canadian traders to operate on the American side of the border, although prior to the war, they and the Americans had easily passed back and forth across the border.