The powerful American gun lobby worries about the reach of a global pact on conventional arms. Delegations from almost 200 countries convened at the United Nations this month to come up with a new treaty that would regulate international trade in conventional weapons. The effort, supported by the Obama administration, has very vocal opposition: the American gun lobby. Proponents of the Arms Trade Treaty argue that inadequate controls over the international arms market result in armed violence against civilians by human rights violators, criminals, gangs, warlords, and terrorists. The treaty would require governments to deny weapons transfers to states that...