During a war in which the United States is a neutral nation, the President, or any person authorized by him, may withhold clearance from or to any vessel, domestic or foreign, or, by service of formal notice upon the owner, master, or person in command or in charge of any domestic vessel not required to secure clearances, may forbid its departure from port or from the United States, whenever there is reasonable cause to believe that such vessel is about to carry fuel, arms, ammunition, men, supplies, dispatches, or information to any warship, tender, or supply ship of a foreign belligerent nation in violation of the laws, treaties, or obligations of the United States under the law of nations. It shall thereupon be unlawful for such vessel to depart.