The United States, the world’s largest arms dealer, has joined more than 90 other nations in signing a treaty that regulates global arms trading, but there is strong resistance in the Senate, which must ratify it. Secretary of State John Kerry, who signed the Arms Trade Treaty on Wednesday, said it was a “significant step” in keeping the world safe and preventing terrorists and others from obtaining conventional weapons. … Addressing U.S. critics of the treaty, the former senator said fears that it would undermine Americans’ constitutional right to keep and bear arms are not grounded in reality. For one,...