Answer + Explanation:
Alrighty, so the belief that the British and Native Americans formed an alliance against the U.S. during the War of 1812 stems from the actions of the British in providing weapons, supplies, and support to Native American tribes in their resistance against U.S. expansion and settlement in the Old Northwest territory. This was seen by the U.S. as a direct threat to their national security and sovereignty, leading to the perception that the British and Native Americans were working together in opposition to U.S. interests.