"The President's request for the W76-2, a supplemental capability, is in response to developments in Russian nuclear doctrine, exercises, and its new nuclear capabilities," Mattis wrote in the June 3 letter obtained by The Washington Post. He called the warhead a "modest adjustment" to the arsenal.
Mattis said the W76-2 "does not require developing a new nuclear warhead or nuclear testing, it does not violate any nuclear arms control treaty, and it does not increase the size of the nuclear stockpile."
The administration plans to modify an existing high-yield warhead, dating to the 1970s, to achieve the low-yield weapon for use on submarines. Congress has backed those plans.
But critics say that even a modification of an existing warhead would needlessly expand US nuclear capabilities at a time when they should be scaled back.
The House and Senate versions of the annual defense policy bill authorise US$65 million to convert some of the high-yield W76-1 nuclear warheads deployed on Ohio-class submarines into the smaller-yield variants known as W76-2 warheads. The House has passed its version of the bill that approves the low-yield variants, which the Pentagon hopes to deploy on Trident D5 ballistic missiles by 2020.
The administration has requested an additional US$23 million in the coming year's budget to flight-test the lower-yield warhead variant on a Trident submarine before deployment. The tests would use a dummy warhead without any fissile material.
Though official US nuclear-warhead yields remain classified, experts estimate that the new W76-2 would explode with a yield of about 6.5 kilotons, whereas the full-size W76 explodes with a yield of roughly 100 kilotons. By comparison, the warheads the US military dropped on Hiroshima and Nagasaki, Japan, in 1945 exploded with about 15 and 20 kilotons of force, respectively.
To put it in perspective, according to Joe Cirincione, the president of the Ploughshares Fund, a non-profit that funds organisation and people working to eliminate nuclear weapons, the explosive yield from the "Mother of all Bombs" or MOAB is around 11 tons, about 44 times the size of a conventional bomb.
While low-yield nuclear bombs can range from .1 kilotons to 10, 20, or 50 kilotons, according to Cirincione, the explosive yield from a low-yield nuclear weapon is equivalent to 1000 MOABs.
"When people talk low-yield, what they generally mean is something that is down like Hiroshima size," Cirincione said. "That is… 'only' 10 thousand tons of explosive force or…1000 MOABs."
The Pentagon's rationale for putting low-yield warheads on submarines rests on the belief that Russian nuclear doctrine has evolved to include the possibility of a battlefield nuclear attack. US defense officials sometimes call the suspected Russian strategy "escalate to de-escalate."
The idea is that Russia - which retains a vast arsenal of small and nimble nuclear arms - could employ one against an American ally or partner in a "limited attack." That would force the United States to choose between responding with a high-yield strategic nuclear warhead, all but guaranteeing full-scale nuclear war, or returning fire with a conventional weapon, risking embarrassment or defeat.
The Trump administration believes the W76-2 would convince Russia that any such attempt would result in a reciprocal low-yield nuclear attack.
The US nuclear arsenal already includes a low-yield nuclear warhead on the B61 gravity bomb, which has an adjustable yield that by some accounts can go as low as 0.3 kilotons. But the US military can launch the B61 and its modifications only from aircraft, which Mattis said in his letter could be "vulnerable to formidable Russian air defenses."
Some experts have questioned whether Russian nuclear doctrine has evolved to include the scenario the administration cites as the rationale for the new warheads. Pentagon officials, however, say Russian military exercises and public statements show the strategy is part of Moscow's playbook.
Mattis sent his two-page document to McConnell in response to a May 22 letter that California Gov. Jerry Brown, D, and prominent former US officials wrote to the Senate majority leader the previous month decrying the W76-2 as a gateway to nuclear catastrophe.
The letter - signed by 32 people, including former secretary of state George Shultz, former defense secretary William Perry and former senator Richard G. Lugar, R-Ind. - called the rationale for the low-yield warheads a "false narrative."
The current US nuclear arsenal, with its roughly 4000 warheads, should leave Russia with little doubt that the United States is serious about deterring a nuclear attack, the letter said, arguing that the president already possesses sufficient low-yield options and flexible responses.
The letter said the introduction of the warheads was based on a "mistaken and dangerous belief" that it would be possible to prevent a limited nuclear exchange between Russia and the United States from escalating into an uncontrollable nuclear war.
Because the US military theoretically could launch a low-yield warhead without causing mass obliteration, the president could be more tempted use one, the critics have argued.
"Ultimately, the greatest concern about the proposed low-yield Trident warhead is that the president might feel less restrained about using it in a crisis," Brown and former officials wrote. "When it comes to using a nuclear weapon, restraint is a good thing."
Mattis, in his rebuttal, rejected the argument out of hand.
"Let me be clear, any decision to employ nuclear weapons would be the most difficult decision a President has to make," Mattis wrote. "This Administration, like the ones before it, has said that nuclear weapons would be employed only in extreme circumstances to protect our vital interests and those of our allies and partners."
Defense officials also note that the president already has low-yield options at his disposal and that the United States had them on submarines until President George H.W. Bush put the military's nuclear-armed Tomahawk cruise missiles in storage and President Barack Obama eliminated them.
Rep. Adam Smith, D-Wash., the top Democrat on the House Armed Services Committee, proposed an amendment to the annual defense policy bill that would have eliminated funding for the warhead and redirected the money to shore up the Army. It failed 33 to 28 in committee along party lines.
Other attempts by Democrats in Congress to stop or roll back the initiative have also come up short. The latest effort - an amendment that Sen. Edward J. Markey, D-Mass., submitted this week as the Senate prepares to vote on the annual defense policy bill - is unlikely to pass and may not receive a vote.
Originally authorised by Obama, the Pentagon's US$1.2 trillion modernisation of the US nuclear triad over 30 years has received bipartisan support, but a Democratic takeover of the House or Senate this November could force a rollback of some of those plans and revive debate about the introduction of the W76-2 warhead.
