In 1862, French forces under Napoleon III invaded and conquered Mexico, giving control to the puppet monarch Emperor Maximilian. Washington denounced this as a violation of the doctrine but was unable to intervene because of the American Civil War. This marked the first time the Monroe Doctrine was widely referred to as a "doctrine." In 1865 the U.S. stationed a large combat army on the border to emphasize its demand that France leave. France did pull out, and Mexican nationalists executed Maximilian.