Even before his most ambitious gun-control proposals were falling by the wayside, President Obama was turning for help to the United Nations. On April 2, the United States led 154 nations to approve the Arms Trade Treaty in the U.N. General Assembly. While much of the treaty governs the international sale of conventional weapons, its regulation of small arms would provide American gun-control advocates with a new tool for restricting rights. Yet because the Constitution requires that two-thirds of the Senate give its advice and consent to any treaty, Second Amendment supporters still have a political route to stop the...