10 USC 351 - During war or threat to national security

(a) The President, through any agency of the Department of Defense designated by him, may arm, have armed, or allow to be armed, any watercraft or aircraft that is capable of being used as a means of transportation on, over, or under water, and is documented, registered, or licensed under the laws of the United States.
(b) This section applies during a war and at any other time when the President determines that the security of the United States is threatened by the application, or the imminent danger of application, of physical force by any foreign government or agency against the United States, its citizens, the property of its citizens, or their commercial interests.
(c) Section 16 of the Act of March 4, 1909 (22 U.S.C. 463) does not apply to vessels armed under this section.