U.S. grapples with risk of nuclear war over Ukraine

• After warning last week that Russia would consider using nuclear weapons, Putin added: "This is not a bluff."
• National Security adviser Jake Sullivan conceded Friday that it might well not be. "There is a risk, given all the loose talk and the nuclear saber-rattling by Putin, that he would consider this," he said.
• Sullivan said the White House had informed the Kremlin of the "decisive" action the U.S. would take if Putin follows through.

Driving the news: Sullivan was responding to an address in which Putin confirmed Russia was extending its nuclear umbrella to four newly annexed territories of Ukraine.

• Putin also said the U.S. had itself "set a precedent" for the use of nuclear weapons when it bombed Hiroshima and Nagasaki.

Between the lines: Putin seems to believe that if he can convince the U.S. and its NATO allies that he's willing to use nuclear weapons over Ukraine, they will pressure Kyiv to surrender, says Alexander Gabuev of the Carnegie Endowment.

• The problem for Putin is the Ukrainian government, military and public believe they're on course for victory, and won't be deterred by nuclear threats, Gabuev says. The U.S., meanwhile, is only increasing its arms shipments to Ukraine.
• But if Ukraine continues to re-take territory Putin has now claimed is part of Russia, he may believe his only option is escalation. Putin's recent moves suggest he "views this war as existential for himself, for his personal survival maybe, for his legacy and for his country," Gabuev says.

Yes, but: We're not yet at the top of the "escalation ladder," Gabuev notes.

• Putin could knock out electricity to major Ukrainian cities, conduct air and missile strikes on particularly sensitive targets, or even use chemical weapons on the battlefield before he reaches for the world's largest nuclear arsenal.
• But Gabuev believes in the last two weeks, the chances of a nuclear strike moved from "single digits" to "double digits."

If Putin does use nuclear weapons, he could seek a "demonstration effect" — perhaps by detonating a nuclear weapon over the Black Sea or in the Arctic — or deploy a smaller-yield "tactical" nuclear weapon on the battlefield, says Andrea Kendall-Taylor of the Center for a New American Security.

• The U.S. would not likely go nuclear in response, but it could conduct a conventional military strike on Russian soil — perhaps targeting the site or unit behind the Russian launch — and pursue non-military steps like permanently seizing Russian central bank reserves, Kendall-Taylor says.

• Rose Gottemoeller, the former Deputy Secretary General of NATO, would expect a "strong non-nuclear" military response, as well as major diplomatic costs for Putin. "He will lose the global south, which for him should be a great shame because he's been very clever at keeping them on his side," she tells Axios.
• Sullivan said the U.S. had privately made clear to the Russians what its responses would be, but "we're not going to telegraph these things publicly." The administration would certainly seek to rally global consequences and condemnation, including from fence-sitting countries like China and India.
• Retired general Ben Hodges, who commanded U.S. forces in Europe, has said if Russia conducts a nuclear attack in Ukraine, the U.S. could destroy Russia's Black Sea Fleet. However, that would risk a Russian counterstrike and potentially all-out war.

What to watch: The battering Russia's military is taking in Ukraine could contribute to further nuclear standoffs, even after the war, Kendall-Taylor says.

• "The more degraded and vulnerable they are in conventional terms, the more they're going to double down on non-conventional things" like nuclear weapons.