The people of the United States, pouring into the West, began to take a more active interest in the fur trade. John Jacob Astor, who had begun as a peddler dealing in trinkets to barter for furs, saw that the Hudson's Bay Company and the Nor'westers were undergoing heavy losses in their strife. He had organized the American Fur Company and amassed a fortune. He went up to Montreal shortly before the War of 1812 and tried to get the NorthWest Company individuals to join him. They were not interested, but he did manage to hire some of the Metis onto his adventures into the Pacific Ocean region. The American Fur Company built Fort Astoria on the Columbia River after a battle with the NorthWest Company. The fur trading direction changed during this period from west/east to north/south, with huge volumes of furs being transported out through Louisiana.

Other American companies had been springing up. Manuel Lisa, a New Orleans Spaniard, began to trade for furs with the Osage Indians around St. Louis. It was he who organized the Missouri Fur Company and so began the leadership of St. Louis in the fur trade. His men traded in the dangerous country of the Blackfeet, near the Three Forks of the Missouri. Each 200 trappers had to be accompanied by an armed force of 50 men to fight the Indians. When Lisa died, in 1820, his men flocked to another organization, later known as "the Rocky Mountain Men." During the War of 1812, between the United States and Great Britain, a British gunboat easily captured Fort Astoria. The ex-Nor'westers, who were in charge of the fort, had no heart in a fight against the British. Astor continued to finance American companies from St. Louis. These companies later split up into American brigades that gave the Hudson's Bay brigades many losing seasons in the Rockies.

In 1816, with the War of 1812 over and a peace treaty signed, the United States Congress ordered all foreign traders off American soil. The NorthWest Company posts in the United States fell to Astor's American Fur Company. In the United States the stream of settlers and trappers pouring into the West had become a flood by 1831. Astor placed Kenneth MacKenzie (an old Nor'wester who resented the union with Hudson's Bay) in charge of Fort Union at the mouth of the Yellowstone. Gradually the country became too tame for the great fur trade of the past. Fort Union passed into the hands of Federal troops, and the trappers of the Rocky Mountain regions grew fewer and fewer. The trade did not disappear from the West for some decades, but the main focus of it shifted far to the northwest after the purchase of Alaska in 1867.

Significant administrative reorganization took place at the time of the amalgamation of the two fur companies of Canada under George Simpson. Simpson, a highly skilled administrator and former NorthWest Company man, was made Governor of the Northern Department, which included the vast territories of Hudson's Bay west to the Rocky mountains. He would become Governor of the Hudson's Bay territory.

On to the end of the Fur Trade