Combine oil, chili powder, salt, cinnamon, coriander, cumin and red pepper flakes in a large saucepan over a medium-high heat.
Heat for 2 minutes, stirring constantly, until spices are fragrant.
Add onion and soy sauce, and saute, stirring frequently, until onions are just translucent.
Add remaining ingredients and stir well to combine.
Heat until mixture reaches a strong simmer, about 5 minutes, then reduce to low, cover and let cook for 25 minutes, or until sweet potatoes are tender.
Stir occasionally to ensure chili doesn't stick or burn.