WASHINGTON - The US Defense Department said on Tuesday the Navy had fielded a low-yield, submarine-launched ballistic missile warhead, something the Pentagon believes is needed to deter adversaries like Russia but which critics say lowers the threshold for using nuclear weapons.Low-yield nuclear weapons, while still devastating, have a strength of less than 20 kilotons. The atomic bomb dropped on Hiroshima, in August 1945, had about the same explosive power. "This supplemental capability strengthens deterrence and provides the United States a prompt, more survivable low-yield strategic weapon," John Rood, the undersecretary of defense for policy, said in a statement."(It) supports our commitment to extended deterrence; and demonstrates to potential adversaries that there is no advantage to limited nuclear employment because the United States can credibly and decisively respond to any threat scenario," Rood added.A 2018 Pentagon document called for the military to expand its low-yield nuclear capability, saying the United States would modify a small number of submarine-launched ballistic missile warheads with low-yield options."The administration's decision to deploy the W76-2 warhead remains a misguided and dangerous one. The deployment of this warhead does nothing to make Americans safer," Democratic Representative Adam Smith, chairman of the House Armed Services Committee, said in a statement.Arms control advocates and some lawmakers have argued that such low-yield weapons reduce the threshold for potentially using nuclear weapons and could make a nuclear conflict more likely. The United States already has air-launched, low-yield nuclear weapons and critics say that should be sufficient."President Trump now has a more usable nuclear weapon that is a dangerous solution in search of a problem," said Kingston Reif, director for disarmament and threat reduction policy at the Arms Control Association advocacy group.The argument for these weapons is that larger nuclear bombs are so catastrophic that they would never be used, meaning they are not an effective deterrent. With less power and destruction, the low-yield option would potentially be more likely to be used, serving as an effective deterrent, military officials have said.The Federation of American Scientists said last week that the Navy was scheduled to deploy the low-yield warhead on the USS Tennessee in the Atlantic Ocean.