Shortly after the Nazi invasion of Poland, as France began preparing for war, Clémenti issued public threats against Édouard Daladier, the Council President. He faced trial for this and was ordered to pay over 4,000 French francs in damages. During the Phony War of September 1939, he was imprisoned for another three months, but ultimately managed to persuade government that he was still loyal, and fought with distinction in the Battle of France. Shortly after the surrender of June 1940, Clémenti became one of the first voluntary Nazi collaborators in occupied France.