Defense Secretary Jim Mattis. (Alex Brandon/AP)

Walter Pincus is a former Washington Post reporter and columnist covering national security issues.

Top Pentagon officials are telling some pretty tall tales in seeking ­congressional support for a new, low-yield, nuclear warhead to put on a long-range, submarine-launched ballistic missile.

Gen. John E. Hyten, commander of U.S. Strategic Command, gave the most unusual rationale when he testified on March 20 before the Senate Armed Services ­Committee.

The stated purpose of this new weapon is to deter the Russians from using any of their low-yield nuclear weapons — something Russian President Vladimir Putin has often threatened to do if he ever found himself being overwhelmed by NATO conventional forces, presumably in Western Europe.

The United States and its NATO allies already have about 200 low-yield nuclear bombs deployed in Europe. But Hyten and Pentagon officials say an additional weapon is needed to deter Putin’s first use of his tactical nukes, because the aircraft that would deliver our bombs, stealthy as they may be, might not be able to get through Russian defenses.

That’s where the new submarine-launched weapon would come in.

In Hyten’s presentation, should the Russians initiate the use of tactical nukes on the battlefield, the United States would launch one or two low-yield weapons from submarines, not toward the battlefield, where allies might be threatened, but toward targets in Russia.

Here’s the most interesting part: How are the Russians going to know the warheads on those incoming missiles are low-yield, and not — like most nuclear warheads delivered by our submarine-launched ballistic missiles — 10 times more powerful than the bombs used to strike Hiroshima and Nagasaki?

Hyten’s initial response to that question was to tell the senators that from launch to detonation some 30 minutes would elapse.

He then explained: “If somebody does detect that launch, they would see a single missile or maybe two missiles coming. They will realize it is not an existential threat to their country and, therefore, they do not have to respond with an existential threat.” By “existential threat” Hyten essentially meant a full-scale first strike by hundreds of U.S. warheads, designed to knock out Russia’s ability to respond and perhaps survive as a nation.

In short, Hyten suggested that Putin — or his successor — would wait 30 minutes for the incoming one or two U.S. missiles to hit Russian targets before deciding whether to launch a major nuclear response back at the United States.

Why does Hyten suggest that?

His answer was surprising: “That is what I would recommend if I saw that coming against the United States.”

Has any prior STRATCOM commander, or any other U.S. senior government official, announced publicly the United States would ride out any nuclear attack before responding?

Hyten went on to explain, “If we do have to respond, we want to respond in kind and not further escalate the conflict out of control.”

He described the new warhead as a “deterrence weapon first, and then a response weapon . . . to keep the conflict from escalating worse. It actually makes it harder for an adversary to use [a nuclear] weapon in the first place and if it does use it, it allows you to respond appropriately.”

Hyten added, “The key is a rational actor. A rational actor is the basis of all deterrent policy.”

Defense Secretary Jim Mattis made a simpler claim for developing the new warhead in testimony on May 9 before the Senate Appropriations defense subcommittee. He described the scenario Hyten used: Russia, facing defeat in a conventional battle, “would escalate to a low-yield nuclear weapon knowing that our choice would be . . . to either respond with a high-yield [nuclear weapon] or surrender — in other words, frankly suicide or surrender, because a nuclear exchange between Russia and the United States would be a disaster for this planet.”

Suicide or surrender are hardly the only choices, and Mattis should know better.

That same day, May 9, Rep. Adam Smith (Wash.), ranking Democrat on the Armed Services Committee, offered the more traditional understanding of how to deter the Russian low-yield nuclear weapon threat. It came during markup of the fiscal 2019 defense authorization bill.

Smith said, “We don’t create this notion that we can just exchange nuclear weapons and as long as they are small it will be okay. It won’t be okay.” Instead, he suggested, the response to the Russians should be, “We have over 4,000 nuclear weapons, and if you launch one, we will launch ours back at you. And we are not going to sit there and be concerned to make sure that ours isn’t bigger than yours when you started this.”

The Washington state congressman added, “If we send that message, that is a very sufficient deterrent.”

The full House Armed Services Committee ended up authorizing $65 million for development of the new low-yield, sub-launched missile and sent the measure on for an eventual vote by the full House. Meanwhile, the Senate Armed Services Committee has scheduled taking up the measure the week of May 21 where it may face more opposition than it did in the House committee. It should.