President Putin’s nuclear saber-rattling today is a reminder, with the election a week away, that the war in Ukraine will be a foreign policy debacle bequeathed by President Biden to his successor. Mr. Putin’s launch of nuclear missiles, simulating a “response to an enemy first strike,” as Reuters puts it, comes as America and its allies in the North Atlantic Treaty are weighing whether to allow Kyiv to fire Western missiles deep into Russian territory.

Plus, too, Mr. Putin has warned the West that allowing Ukraine to launch such strikes “would put NATO at war with his country,” as the Associated Press puts it. The Russian tyrant, moreover, revised the Kremlin’s nuclear doctrine. Russia will now deem a “conventional attack on Russia by a nonnuclear nation that is supported by a nuclear power,” the AP reports, “to be a joint attack on his country.”

That’s the context in which President Biden has held back from giving President Zelensky permission to use Western munitions to strike deep into the Russian homeland. Ukraine has nonetheless managed to hit targets inside Russia and hundreds of miles from its border with Ukraine, as our James Brooke has reported. Targets have included ammunition depots — and even vodka distilleries — using improvised weaponry like long-range drones. 

These attacks by Kyiv amount to black eyes rather than knockout blows against Russia. Even so, Mr. Putin’s own inability to win the war he started is a testament to the doughtiness of the Ukrainians. Mr. Putin’s importing of North Korean troops marks the war’s strain on Russia’s economy and its available manpower, as the Institute for the Study of War reports, noting “increasingly acute challenges to Putin’s ability to sustain the war over the long term.”

The prospect of a deadlock, no doubt, animated Mr. Zelensky’s plea for advanced missiles — and his ill-advised flirtation with American electoral politics — as much as it inspired Mr. Putin’s atomic scare tactics. The Foundation for Defense of Democracies’ Ivana Stradner, writing in these pages, calls “nuclear blackmail” Mr. Putin’s most effective “tactic to successfully” deter America “from scaling up military assistance.” Who has the nerve to call that bluff?

Not, it would appear, President Trump, by his own account. During the presidential debate he called the conflict “a war that’s dying to be settled,” and pointed to Mr. Putin’s nuclear arsenal. “Nobody ever thinks about that,” Mr. Trump noted. “And eventually, uh, maybe he’ll use them. Maybe he hasn’t been that threatening. But he does have that. Something we don’t even like to talk about. Nobody likes to talk about it.”

Vice President Harris vows to extend Mr. Biden’s policy, “to preserve the ability of Zelenskyy and the Ukrainians to fight for their independence.” Critics contend this policy amounts to giving Kyiv “just enough” aid to keep Russia at bay — not to win the war. No wonder Mr. Zelensky is speaking of rebuilding Kyiv’s own nuclear arsenal, after Ukraine in 1994 gave up its A-bombs on a pledge by America and other Western powers to respect its borders.

That transaction, known as the Budapest Memorandum, is likely to go down as one of the worst bargains ever struck in the history of diplomacy. Thirty years later, President Clinton, one of the signers of the pact, expresses his regrets, Mr. Brooke reports. “I feel a personal stake because I got them to agree to give up their nuclear weapons,” he laments. It’s a cautionary tale for those who put faith in high-minded multilateral agreements or the goodwill of nations.