Memorials in Hiroshima and Nagasaki eloquently testify that nuclear weapons are not simply a bigger, better version of conventional explosives. Yet the haze of passing time seems to have dulled congressional understanding of the ghastly difference.

Last week, the Senate bowed to Bush administration wishes and voted to repeal a 10-year-old congressional ban on the development of small nuclear weapons for tactical use on battlefields. The Senate also gave preliminary approval to $15 million for further research on a nuclear “bunker-buster” that would explode underground with yields far greater than the bomb dropped on Hiroshima. It allocated more millions for nuclear testing, in case the Pentagon decides to resume the program suspended by President Clinton. That authorization vote, to be confirmed in later appropriations bills, puts the United States on a backward path.

The Pentagon says research is not the same as development, testing, deployment or use. All true. But once a new weapon is developed, pressure to test it and then to verify that it actually works in battle becomes great.

The military’s trumpeted success with existing precision weapons in Afghanistan and Iraq undercuts arguments that more nuclear arms are needed. Consider what the reaction of Iraqis and the world would have been if one of the precision bombs aimed at Saddam Hussein had been a small nuke.

Washington should not be showing the way to new atomic weapons that are easier to use. Anything that spurs nuclear competition will increase the number of bombs — and bomb developers — that can fall into the hands of an Al Qaeda. Beyond that, the world should fear an arms race that produces more nuclear weapons in perennial enemy states like India and Pakistan and unpredictable nations like North Korea.

The United States has disposed of most of its smaller, tactical nuclear weapons and has agreed with Russia to destroy many larger strategic ones as well. Pledging support for the Nuclear Nonproliferation Treaty, as the administration does, is inconsistent with developing new weapons.

The administration’s Nuclear Posture Review of 2001 was decried for reviving possible U.S. use of nuclear weapons against nonnuclear countries and raising the possibility of preemptive attacks on any nation developing weapons of mass destruction. The review, a periodic updating of U.S. nuclear strategy, and the Pentagon’s weapons request make the world more dangerous, not less. The nuclear genie has been out of the bottle since 1945. Continued control of its spread should be a hallmark of U.S. policy.