The treaty also has important verification requirements, like semi-annual data exchanges on the two nations’ weapons systems. Both agreed to notify each other of certain nuclear-related actions, and can conduct up to 18 inspections annually of the other’s strategic forces. Mr. Obama knew how essential the treaty was, as did the Senate, which ratified it 71 to 26. It also had the unanimous support of the Joint Chiefs of Staff, and seven former military commanders who had controlled the strategic nuclear forces.

If Mr. Trump is foolish enough to leave the treaty, the United States and Russia will be free to build up arsenals that have declined by thousands of weapons since the late 1960s. That would set off a costly, destabilizing arms race. And by eliminating verification and transparency requirements, America would lose insight into Russia’s program.

Mr. Trump not only seems inclined to undermine New Start limits; he has said he wants to ensure that America’s arsenal is at the “top of the pack.” It is already ahead of the pack with more than enough nuclear weapons, backed by advanced conventional weapons, to keep the country safe.

When stockpiled warheads are factored in, the United States and Russia have roughly 4,500 warheads each, and both are engaged in modernization programs. The next-largest arsenals are France’s, at 300 warheads, and China’s, at 260. A 2013 Pentagon study said America could maintain a strong and credible deterrent with 1,000 warheads.

After refusing to engage in new nuclear negotiations with the Obama administration, the Russians are signaling a willingness to extend New Start when it expires in 2021. Some Republicans are opposed, either because they have an ideological aversion to any restraints on the military or because they want to block an extension in retaliation for Moscow’s deploying a new nuclear-capable cruise missile, a deployment that violates a different treaty.