'Of course' the US should respond with force if Russia uses nukes or chemical weapons in Ukraine, former US ambassador to NATO says

• The war in Ukraine is not going well for Russia and there are growing concerns Putin could use a weapon of mass destruction.
• This could pull NATO and the US into the war. Biden said the alliance "would respond" if chemical weapons are used.

The US and NATO have consistently made it clear they have no interest in sending troops into Ukraine. But if Russian President Vladimir Putin uses a weapon of mass destruction, it could pull the alliance into a war it's been desperate to avoid direct involvement in.

President Joe Biden has repeatedly said US forces would stay out of Ukraine, at one point warning a direct confrontation would be "World War III." Similarly NATO Secretary General Jens Stoltenberg has underscored that the alliance does not want to send in troops.

But the war is going poorly for Russia. There are growing concerns that Putin will use a chemical or nuclear weapon in Ukraine to try to breakout of the bloody stalemate his forces are in. And that may compel NATO's involvement.

"Putin...shows no signs of backing down. The Ukrainians are fighting back with courage and determination, and as long as the war continues, the use of nuclear weapons will be a real possibility," Scott D. Sagan, a political scientist at Stanford University, wrote in a recent op-ed for Foreign Affairs.

Former US Ambassador to NATO Ivo Daalder told Insider that the US and NATO "should help the Ukrainians, and beat the Russians" in such a scenario. "We can't stand by as Russia decides that it's going to use chemical warfare...or a nuclear weapon and say, 'That's none of our business,'" he added.

Biden on Thursday warned that the US and NATO "would respond" if Putin employs a chemical weapon in Ukraine. "The nature of the response would depend on the nature of the use," he added. And Stoltenberg on Wednesday warned, "Any use of chemical weapons would fundamentally change the nature of the conflict."

Russia has faced crippling sanctions as a result of the war, and has been isolated economically and politically. NATO estimates that the Russian military has already seen up to 40,000 battlefield casualties in just a month of war. Putin could feel pressured into doing something drastic — including using a weapon of mass destruction — if he feels it will help Russia gain leverage.

Since launching the war, Putin has made nuclear threats. The Russian leader ordered his country's nuclear deterrent forces on high alert. Kremlin spokesperson Dmitry Peskov also refused to rule out the use of nuclear weapons in a recent interview with CNN.

UN Secretary-General António Guterres in mid-March said nuclear war is "back within the realm of possibility" because of Putin's actions.

'Of course we should use force'

With Russia engaging in what Stoltenberg has decried as "nuclear sabre-rattling," congressional lawmakers are increasingly discussing how the US should respond in the event that nuclear or chemical weapons are used. The US has consistently warned Russia it will defend every inch of NATO territory, signaling to Moscow that any attack on an alliance country would lead to a military response.

Along these lines, some members of Congress have suggested that if Russia uses a nuclear weapon in Ukraine (which is not a NATO member) and the radiation from the attack drifts across the border into a NATO country like Poland, it should be considered an attack on the alliance and trigger a military response. NATO operates under the principle of collective defense, considering an attack on one an attack on all, which is enshrined in Article 5 of its founding treaty.

"If a nuclear device is detonated and the radiation goes into a [neighboring] country, that could very well be perceived as an attack against NATO," Democratic Sen. Jack Reed of Rhode Island, chairman of the Senate Armed Services Committee, told reporters earlier this week, per Military.com. Reed added that this could also be true regarding "some chemical, biological attacks."

Similarly, GOP Sen. Marco Rubio of Florida, the vice chairman of the Senate Intelligence Committee, told NBC News that Russia using a nuclear or chemical weapon could force NATO members to invoke Article 5 and lead the alliance to retaliate. "As you detonate a nuclear weapon inside of Ukraine depending on what it is they detonate, even in a demonstration, that would spread radioactive material that would cross borders potentially," Rubio said, adding, "If radioactive material blows across the Polish border, they would argue they've been attacked."

But Daalder said that invoking Article 5 is not necessary for the alliance to use force, citing the NATO's 2011 intervention in Libya as an example. NATO doesn't have to wait for a chemical or radioactive clouds to cross over into Poland or another member country to respond, he said, there just needs to be a consensus among allies — and no objections — for force to be employed.

"You don't need Article 5 to use force," Daalder told Insider, adding that if Russia uses a weapon of mass destruction "of course we should use force."