“Our deterrence is much, much more than just nuclear weapons,” Adm. Haney said in a lecture at Kansas State University just days before Global Thunder 17 started. “If necessary,” he said, the United States “will respond at a time and place and domain of our choosing.”

The Iran scenario has never before been publicly divulged. All STRATCOM says of the 2016 war game is that it followed “a notional, classified scenario.”

Though the United States has never made any public or explicit nuclear threat against Iran, in the past year, it has deployed a new nuclear weapon which increases the prospects for nuclear war. The new nuclear weapon, called the W76-2, is a “low yield” missile warhead intended for exactly the type of Iran scenario that played out in the last days of the Obama administration. Military sources directly involved in nuclear war planning say there has been no formal change in war plans with regard to Iran under the Trump administration, but the deployment of what they say is this “more usable” weapon, changes the nuclear calculus.

In exclusive reporting for Newsweek, four senior military officers say they doubt that the now six-month standoff with Iran could escalate to nuclear war. But they each note the deployment of the new Trident II missile warhead explicitly intended to make the threat of such an attack more credible, and point it out as a little understood or noticed change that increases the very danger. They argue that the new capability should give Tehran pause before it contemplates any major attack on the United States or its forces. But all four also add, very reluctantly, that there is a “Donald Trump” factor involved: that there is something about this president and the new weapons that makes contemplating crossing the nuclear threshold a unique danger.

Nuclear weapons have been a part of military contingency plans dealing with Iran going back to the George W. Bush administration’s 2002 Nuclear Posture Review. In its guidance to nuclear war planners just three months after 9/11, the White House added the “axis of evil” states (Iraq, Iran, North Korea) plus Syria and Libya to Strategic Command’s missions.

After much internal debate, President Barack Obama wrote his own Nuclear Posture Review that affirmed there were “a narrow range of contingencies”—either to deter a massive conventional attack or to stop enemy use of chemical or biological weapons—where the United States might use nuclear weapons first and even against non-nuclear nations, precisely the scenario that later played out in Global Thunder 17. According to partially declassified documents obtained by the Federation of American Scientists, new nuclear war plans written in the Obama administration formally included Iran.

Hans Kristensen of the Federation points out that this is the state of affairs inherited by Donald Trump. National policy affirmed by two previous administrations includes the possibility of nuclear use against Iran, while the experience in war gaming such scenarios—and not just against Iran—exposed weaknesses in the ability of Strategic Command to carry out such a presidential order. Thus emerged the “requirement” on the part of the military to create a new weapon to fulfill this first strike scenario.

“Regardless of presidencies, nuclear planning tends to have a life of its own,” Kristensen said in an interview last week, adding that “Iran is very much in the crosshair.” That’s because, as Kristensen notes, nuclear planners operate from “relatively vague presidential guidance,” writing scenarios, conducting war games, and adjusting plans, weapons and the posture of forces to anticipate countless possible scenarios.

When Donald Trump became president, one of his first acts was signing a memorandum on “Rebuilding” U.S. armed forces. That memorandum directed his new Secretary of Defense, retired Gen. James Mattis, to initiate a new Nuclear Posture Review and to ensure that the nuclear deterrent was “ready and appropriately tailored to deter 21st-century threats.” Strategic Command had already determined that it needed a new nuclear weapon to deal with advanced and emerging nuclear powers like North Korea and Iran. Now they had their marching orders.

“They answered their own mail,” one retired Air Force officer involved in the early Trump White House said of the national security directive.

Inside the nuclear establishment, “appropriately tailored” meant a new small nuclear weapon, one deliverable by a ballistic missile rather than from a bomber. The latter, as was gamed in the Global Thunder exercise, would take 11 hours to fly from home base in Missouri to either Iran or North Korea. A missile, on the other hand, could take 30 minutes, and a forward-deployed submarine-launched missile could take just 10-15 minutes.

North Korea’s string of long-range missile tests in the first year of the Trump administration accentuated this “hole” in U.S. nuclear capabilities, a senior Air Force officer involved in the nuclear deliberations says. In the most pressing scenario involving the imminent use of weapons of mass destruction, existing missiles were rejected as a credible deterrent threat because their warhead size was thought to be too large to be “usable.”

In the rarified world of nuclear war planning, only a single small nuclear weapon launched from a Trident submarine represented the credible and “prompt” capability needed to respond to new threat. That is, a new nuclear weapon that could actually be used to preempt a strike on the United States or its Asian allies. B-2 bombers in theory could be forward deployed with nuclear bombs to shorten response time, but such a forward deployment had never been tried, and would demand consultation with and permission from allies. War planners concluded that even there, that a bomber mission would take hours—not fast enough—and there was a possibility that a bomber might be shot down.

In February 2018, the Trump administration concluded its own Nuclear Posture Review.

“We must look reality in the eye and see the world as it is, not as we wish it to be,” Defense Secretary Mattis wrote in the introduction.

The Review formally called for a new low-yield warhead to be deployed on Navy Trident II submarine-launched missiles. Though articulated as a counter to Russia, government and non-government officials today agree that the new W76-2 warhead was all along also intended to fill the niche of providing a usable and prompt weapon to counter imminent North Korean or Iranian attacks, either with WMD or long-range missiles.

In late January 2019, with little fanfare, the first of these low-yield W76-2 nuclear warheads started rolling off the Department of Energy production line in Amarillo, Texas. In September, according to officials who spoke on background because no announcement has been made, the first W76-2 warheads were delivered to the Navy. That W76-2 is thought to have an explosive yield of between 5-6 kilotons (5-6 thousand tons) – about one-third the size of the atomic bomb dropped on Hiroshima. Kristensen estimates that some 50 of these small, “prompt” warheads will be deployed on Trident submarines, and that two of the 24 missiles aboard each of 12 submarines will be so armed.

On October 30, 2016, a day before Global Thunder 17 ended, the USS Pennsylvania, a Trident ballistic missile submarine based in Washington state, surfaced in Apra Harbor, Guam. It was the first visit of a ballistic missile submarine to Guam in 28 years and only the third Trident submarine to make a foreign port visit since 9/11.

“This visit is a clear demonstration of the highly survivable and lethal capabilities the United States brings to bear in support of the unwavering extended deterrence commitments to our allies,” said Adm. Harry Harris, then the commander for U.S. Pacific Command (and now U.S. ambassador to South Korea).

The voyage of the USS Pennsylvania was an introduction to its unique and expanded “tactical” duty, one that was now extending the mission of nuclear submarines beyond Russia and China.

Nine months later, another ballistic missile submarine, the USS Kentucky, showed up off Dutch Harbor in the Aleutian Island chain of Alaska, just 3,400 miles from its North Korean targets.

Trident submarines rarely surface once they leave their ports, operating on 100-day cycles, about 70 days underwater followed by 30 days replenishment before a new crew takes over. Since Donald Trump has become president, though, four Trident submarines have surfaced during their patrols, the two in the Pacific and two others in the Atlantic, both making port calls in Scotland.

To conduct visible nuclear diplomacy, the U.S. military relies on its 156 strong bomber force – the B-2 Spirit stealth bombers, the venerable B-52 Stratofortress bombers, and even the conventional-only B-1 Lancer bomber.

Last May, as the Trump administration began accelerated military deployments “in response to a number of troubling and escalatory indications and warnings” from Iran, bombers played a visible role. B-52 bombers were deployed to an airbase in Qatar, on the Persian Gulf, for two months. And at the end of October, B-1 bombers flew from South Dakota all the way to Saudi Arabia, the first time heavy bombers were on the ground in that country since Operation Desert Storm in 1991.

But then bombers more or less disappeared from Middle East skies. Global Thunder 20, this year’s nuclear exercise, completely focused on a Russia scenario. The scenario for the October 2019 exercise had been selected more than a year before.

Last week, six B-52 bombers showed up on the Indian Ocean island of Diego Garcia, the first time bombers have forward based to the British-controlled territory in more than a decade. Retired Air Force Gen. Hawk Carlisle told Air Force Times that the placement of the bombers 3,000-plus miles from the southern edge of Iran put them out of range of Tehran’s medium-range ballistic missiles.

None of these forward deployed bombers have nuclear weapons with them, nor are there nuclear weapons deployed at the half dozen forward bomber bases used in the Pacific, Europe or the Middle East. If there was any conceivable American nuclear strike on Iran, sources agree, it would come from the new low-yield Trident submarine-based system.

No one in the Air Force or Strategic Command wanted to talk on the record regarding nuclear plans or the actual prospects of nuclear weapons playing a role in the ongoing Iran crisis, cautious in speaking of highly classified war plans and mindful of the president’s operating style.

On the philosophical question of using nuclear weapons, all six Air Force and STRATCOM sources I spoke to expressed concern that the very existence of nuclear options, with this president, complicated their otherwise clear conviction that there was no way nuclear weapons could be used against Iran. American nuclear use could only occur, they agree, after the countries were in a full-scale war, and after the Iranian use of chemical or biological weapons or after a direct attack on the United States. And even then, they say, a nuclear option might only be discussed were there unmistakable intelligence that Tehran was preparing an imminent strike with some kind of improvised radiological or other weapon of mass destruction.

In such a scenario, these officers agree, the president’s decision-making could be both opaque and unpredictable. In July, when Trump was offered the option of striking Iranian air defense targets in retaliation for the downing of an unmanned reconnaissance drone, he rejected even a very limited option, concerned that 150 civilians might die in the attacks. But the president chose the most extreme option in the January 2 drone strike in Baghdad that killed Iranian Gen. Qassem Soleimani.

One retired Air Force officer told me this week that what worries him is that a “package” of American options in response to the most extreme Iranian actions will automatically include the nuclear option, even if it is one option out of a hundred. Having a “prompt low collateral damage W76,” the officer says, connotes a usable nuclear weapon.

That’s what the nuclear war planners created, based upon presidential guidance.

As the current nuclear war plans are written, the use of such a weapon could also be justified, almost Hiroshima-like, as a shocking thunderclap to forestall a wider and theoretically more destructive all-out war.

“It is a capability that the United States did not have a year ago,” the officer says, built precisely to be used, even to be used preemptively. “Let’s just hope that that option is never offered.”

William Arkin is author of a half-dozen books on nuclear weapons. He is writing Ending Perpetual War for Simon & Schuster. His Twitter handle is @warkin