Except in those cases in which the exportation of arms and munitions of war or other articles is forbidden by proclamation or otherwise by the President, as provided in section 401 of this title, nothing herein contained shall be construed to extend to, or interfere with any trade in such commodities, conducted with any foreign port or place wheresoever, or with any other trade which might have been lawfully carried on before June 15, 1917, under the law of nations, or under the treaties or conventions entered into by the United States, or under the laws thereof.