As concerns grow over Russian President Vladimir Putin's nuclear saber-rattling amid continued losses in Ukraine, what a U.S. response would look like has become an increasingly urgent question.
U.S. officials since the start of Russia's attack on Ukraine have stressed there are plans being developed to counter a range of moves by Moscow but have kept specifics under wraps.
While the administration says there are no signs that the Kremlin has made moves toward a nuclear strike - and that Washington has not changed its own nuclear position - experts say the potential U.S. options could turn into a very real scenario given Russia's floundering military campaign and an increasingly frustrated Putin.
Mark Cancian, a former Pentagon official-turned-defense expert at the Center for Strategic and International Studies, said a U.S. response to a major Russian attack would be twofold - one military and one diplomatic.
"If the Ukrainians kept fighting, we would continue our flow of aid and we'd probably take the gloves off" in terms of weapons provided to Kyiv, he told The Hill.
At the top of Ukraine's wish list is the Army Tactical Missile System (ATACMS), a surface-to-surface missile that can travel four times farther than anything Kyiv has now in its fight against Russia. The embattled country has pressed the U.S. for the system for months, but Washington has been hesitant to provide it over fears it could escalate the conflict.
However, should Moscow use a tactical nuclear weapon on Ukrainian troops or civilians, or even detonate such a device away from populated areas, Cancian predicted the administration would finally allow Kyiv to have ATACMS or "anything else they wanted" to go after Russian targets.
On the diplomatic side of things, meanwhile, Russian use of nuclear weapons could very well prompt countries such as India, China and Turkey - the latter a NATO ally - to put pressure on Putin economically, according to Cancian.
"A nuclear strike would really, I think, put them under a lot of pressure to go along with the sanctions and take a tougher line towards Russia, so Russia would lose these lifelines that they've been clinging to and nurturing," he said.
National security adviser Jake Sullivan last week said there would be "catastrophic consequences" should Moscow deploy nuclear weapons and said a more specific ultimatum had been delivered to Moscow privately.
President Biden has said since the start of the war that U.S. troops will not be sent to Ukraine, and experts warn that a nuclear response to a nuclear attack could quickly escalate into a nuclear world war.
Retired Gen. David Petraeus offered a prediction of how the U.S. would respond to a Russian nuclear attack on Sunday, though he noted that he had deliberately avoided speaking with Sullivan about it.
"I mean, just to give you a hypothetical, we would respond by leading a NATO, a collective effort, that would take out every Russian conventional force that we can see and identify on the battlefield in Ukraine and also in Crimea and every ship in the Black Sea," he said.
Laura Cooper, the deputy assistant secretary of Defense for Russia, Ukraine and Eurasia, said Tuesday that U.S. officials "have continually consulted with allies about the Russia threat, and the nuclear threat that Russia poses is just one aspect of that, and certainly the NATO forum is our premier forum for consultation on these issues."
One Austrian official told The Hill that it's offered the country as a neutral ground for difficult negotiations and is ready to host de-escalation talks and maintain channels with Russia.
Though Putin's national televised speech last month was not his first time raising the specter of nuclear war, current and former U.S. officials have raised new alarms over the Kremlin's increasingly bellicose nuclear rhetoric as it moves to annex four regions of Ukraine.
Putin threatened on Aug. 21 that Moscow would deploy its massive nuclear arsenal to protect Russian territory or its people - which could now include the four Russian-occupied Ukrainian regions. However, both Kyiv and Washington have said they will not be deterred from continued fighting to take back those regions.
Defense Secretary Lloyd Austin, in an interview with CNN aired Sunday, said that while he hasn't seen intelligence to suggest the Russian leader has chosen to use nuclear weapons, "there are no checks on Mr. Putin."
"To be clear, the guy who makes that decision, I mean, it's one man," Austin said.
John Kirby, the National Security Council's coordinator for strategic communications, said Monday that the U.S. is "closely" watching Russian activity at the Zaporizhzhia power plant - another location Putin could choose to attack to escalate the war.
And former national security adviser H.R. McMaster on Sunday said Putin is "under extreme pressure" due to battlefield failures and domestic outcry over a mobilization order that could send hundreds of thousands of reservists into the war.
"I think the message to [Putin] is If you use a nuclear weapon, it's a suicide weapon. And the response from NATO and the United States doesn't have to be nuclear," McMaster told "Face the Nation" host Margaret Brennan on CBS.
Fears were further stoked this week when an online video emerged of a train in Russia appearing to carry equipment from a Kremlin military unit that handles nuclear weapons. The video, which Pentagon officials could not confirm, shows military vehicles allegedly from the secretive 12th Main Directorate of the Russian ministry of defense being transported on the train, according to Konrad Muzyka, an aerospace and defense analyst focused on Russia and Belarus.
The Kremlin unit is responsible for nuclear munitions, their storage, maintenance, transport and issuance, Muzyka tweeted Sunday.
"I have seen these reports. I have nothing to corroborate," Cooper told reporters Tuesday when asked about the video.
Pressed on whether the Pentagon has seen anything to indicate that Russia is contemplating the use of nuclear weapons, she said officials "have certainly heard the saber rattling from Putin" but "see no signs that would cause us to alter our posture."
Cooper also declined to answer questions on whether the U.S. has seen any movement of Russia's nuclear forces, citing the protection of U.S. intelligence.
Some, including Rep. Mike Turner (R-Ohio), ranking member of the House Intelligence Committee, have urged the administration to increase its nuclear readiness in Europe and move additional missile defense assets into the region.
"This administration needs to step up its game on missile defense," Turner said on Fox News over the weekend. "We have assets in Europe, and we need to engage them so that we can provide protection to our allies."
Much speculation has also been given as to the exact kind of weapon Putin might potentially use, with fears he could resort to using tactical nuclear weapons - meant to be used in a battle or on a specific population center to try to bring an end to the conflict.
"We always have to try to take the threat of nuclear use seriously and so we do, and that's why we are watching very closely, and that's why we do consult closely with allies," Cooper said.
"But at the same time, at this point, [Russia's] rhetoric is only rhetoric, and it's irresponsible saber-rattling that we see at this point," Cooper said.
For now, the U.S. will respond to Russian aggression by continuing to pour weapons and other aid into Ukraine, including four more of the advanced rocket systems Kyiv has credited with greatly helping its offensive begun at the start of this month.
The soon-to-be delivered High Mobility Artillery Rocket Systems - used by the Ukrainians to target bridges, roads and munition storage areas Russia uses to supply its forces - are part of a new $625 million lethal aid package announced Tuesday.
Asked later on Tuesday whether the United States will provide anything to help the Ukrainians protect themselves against a possible nuclear strike, Cooper said Washington has already provided "a considerable amount of protective equipment against chemical, biological and radiological threats."
She pointed to a military aid package from earlier this year that included "a number of personal protective equipment items" as well as "significant quantities" of such equipment given as part of the Cooperative Threat Reduction Program.
Laura Kelly contributed reporting.
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