Defense Secretary Jim Mattis told Congress in June that he was open to reconsidering the need for both systems. But in remarks to sailors in Washington State almost three weeks ago, he hinted at where a nuclear review was going to come out.

“I think we’re going to keep all three legs of the deterrent,” he told the sailors.

The contracts, and Mr. Mattis’s hints about the ultimate nuclear strategy, suggest that Mr. Obama’s agreement in 2010 to spend $80 billion to “modernize” the nuclear arsenal — the price he paid for getting the Senate to ratify the New Start arms control agreement with Russia — will have paved the way for expansions of the nuclear arsenal under Mr. Trump.

“It’s been clear for years now that the Russians are only willing to reduce numbers if we put limits on missile defense, and with the North Korean threat, we can’t do that,” said Mr. Samore, who is now at Harvard’s Belfer Center for Science and International Affairs. “I think we are pretty much doomed to modernize the triad.”

At issue in the debate over the cruise missile and the rebuilding of the land-based fleet is an argument over nuclear deterrence — the kind of debate that gripped American national security experts in the 1950s and ’60s, and again during the Reagan era.

Cruise missiles are low-flying weapons with stubby wings. Dropped from a bomber, they hug the ground to avoid enemy radars and air defenses. Their computerized brains compare internal maps of the terrain with what their sensors report.

The Air Force’s issuing last week of the contract for the advanced nuclear-tipped missile — to Lockheed Martin and Raytheon Missile Systems — starts a 12-year effort to replace an older model. The updated weapon is to eventually fly on a yet-undeveloped nuclear bomber.

The plan is to produce 1,000 of the new missiles, which are stealthier and more precise than the ones they will replace, and to place revitalized nuclear warheads on half of them. The other half would be kept for flight tests and for spares. The total cost of the program is estimated to be $25 billion.