It is hardly the first time the United States has seen no inconsistency in expanding its own nuclear capabilities while trying to persuade lesser powers to give up theirs.

In fact, the imbalance is built into the Nuclear Nonproliferation Treaty, which went into effect in 1970. It prohibits all states that did not already have the bomb from building nuclear weapons. (Israel, Pakistan and India never joined, and North Korea dropped out.)

But it also requires the acknowledged nuclear powers — the United States, Russia, China, Britain and France — to work toward “the cessation of the nuclear arms race and to nuclear disarmament,” and ultimately to complete their own disarmament.

For the two decades after the fall of the Berlin Wall, both the United States and Russia could argue that they were making progress on that promise. The number of nuclear weapons deployed by the two countries fell, and fell again, under a series of arms control agreements, and as of earlier this year, both are now limited to 1,550 deployed weapons. Thousands more are in storage.

President Barack Obama argued that the United States could not urge other countries to give up nuclear programs while expanding its own. But many of his own aides later said they wished he had done far more to reduce America’s arsenal, arguing that it could safely drop below the number the Russians deployed.

Now Mr. Trump is heading in the other direction. The United States has dramatically stepped up the effort to overhaul the existing arsenal and prepare for the day when it might once again be enlarged. Unless the New Start Treaty is renewed for five years, any limits on the American and Russian arsenals will expire in February 2021, just days after Mr. Trump would enter his second term.