Whoever, when the United States is at war, willfully causes or attempts to cause insubordination, disloyalty, mutiny, or refusal of duty, in the military or naval forces of the United States, or willfully obstructs the recruiting or enlistment service of the United States, to the injury of the service or the United States, or attempts to do so—

Shall be fined under this title or imprisoned not more than twenty years, or both.