Public opinion pressured the U.S. government to bring Villa to justice for the raid on Columbus, New Mexico; U.S. President Wilson sent Gen. John J. Pershing and some 5,000 troops into Mexico in an unsuccessful attempt to capture Villa. It was known as the Punitive Expedition. After nearly a year of pursuing Villa, American forces returned to the United States. The American intervention had been limited to the western sierras of Chihuahua. Villa had the advantage of intimately knowing the inhospitable terrain of the Sonoran Desert and the almost impassable Sierra Madre mountains and always managed to stay one step ahead of his pursuers. In 1923 Villa was assassinated by a group of seven gunmen who ambushed him while he was sitting in the back seat of his car in Parral.