 Because Mexico City lacked walls and was essentially indefensible, Santa Anna sought to defeat Scott in a pitched battle, choosing to mount a defense near the Churubusco River, several miles south of the city.The Battle of Contreras began on the afternoon of August 19, when the Mexican army under General Gabriel Valencia attacked and pushed back an American detachment charged with building a road.In the early morning of the following day, an American force led by General Persifor Frazer Smith surprised and decimated Valencia's army.