19 CFR § 4.73 - Neutrality; exportation of arms and munitions.
(a) Clearance shall not be granted to any vessel if the port director has reason to believe that her departure or intended voyage would be in violation of any provision of the Neutrality Act of 1939 or other neutrality law of the United States, 104 or of any regulation or instruction issued pursuant to any such law.
(b) The port director shall refuse clearance for and detain any vessel manifestly built for warlike purposes and about to depart from the United States with a cargo consisting principally of arms and munitions of war 105 when the number of men intending to sail or other circumstances render it probable that the vessel is intended to commit hostilities against the subjects, citizens, or property or any foreign country, with which the United States is at peace, until the decision of the President thereon is received, or until the owners shall have given bond or security in double the value of the vessel and its cargo that she will not be so employed.
105 Clearance for vessel shall not be denied for the sole reason that her cargo contains contraband of war.
(c) A port director shall promptly communicate all the facts to Headquarters, U.S. Customs Service, if he learns while the United States is at peace that any vessel of a belligerent power which has arrived as a merchant vessel is altering, or will attempt to alter, her status as a merchant vessel so as to become an armed vessel or an auxiliary to armed vessels of a foreign power.
(d) If a port director has reason to believe during the existence of a war to which the United States is not a party that any vessel at his port is about to carry arms, munitions, supplies, dispatches, information, or men to any warship or tender or supply ship of a belligerent nation, he shall withhold the clearance of such vessel and report the facts promptly to Headquarters, U.S. Customs Service.