Final answer:
Tecumseh sided with the British in the War of 1812 to resist the United States, which he viewed as an enemy due to the threat of land loss and cultural erosion it posed to Native American tribes.
Step-by-step explanation:
The best explanation for why Tecumseh fought alongside the British in the War of 1812 is that c. Tecumseh considered the United States to be an enemy. Tecumseh's aim was to preserve his people's lands and culture by resisting American expansion. He and his followers, including the Red Sticks, formed an alliance to reject white influence, return to their ancestral ways, and defend against American encroachments. This objective aligned with British interests, as most Indian tribes and nations supported the British due to fear of losing more land to American expansion should the United States be victorious.