The White House divides the alliance and signals weakness to Putin by refusing to let Warsaw send fighter jets to Ukraine.

The U.S. and Europe have shown impressive cohesion since Vladimir Putin’s invasion of Ukraine, which makes this week’s fiasco over delivering Poland’s MiG fighters to Kyiv so damaging. The message to Mr. Putin is that his intimidation works and NATO can be divided.

On Tuesday Poland said it could transfer around two dozen MIG-29 jet fighters to a U.S. base in Germany, and then to Ukraine, whose pilots can fly the Soviet-era planes with minimal training. On Sunday Secretary of State Antony Blinken had said the U.S. was working with the Poles on the issue and would try to “backfill anything that they provide to the Ukrainians.” Yet Washington later claimed surprise at Poland’s proposal.

“The decision about whether to transfer Polish-owned planes to Ukraine is ultimately one for the Polish government,” said a Pentagon spokesman in a statement late Tuesday. “The prospect of fighter jets ‘at the disposal of the Government of the United States of America’ departing from a U.S. NATO base in Germany to fly into airspace that is contested with Russia over Ukraine raises serious concerns for the entire NATO alliance.” He added that the plan lacked “substantive rationale” and was not “tenable.”

Untenable how? After a NATO no-fly zone, which the alliance has refused, the MiGs are Ukrainian President Volodymyr Zelensky’s top request. The jets won’t decide the war, but his generals must think they’d help if only to deny Russia control of the skies. Any Russian artillery batteries or jets taken off the battlefield could save Ukrainian lives.

What happened between Mr. Blinken’s endorsement and the Pentagon’s rejection? It’s hard not to conclude that the White House blinked for fear of provoking Mr. Putin, who is demanding that the West stop arming Ukraine.

But NATO countries are already sending all sorts of weapons into Ukraine. Is a Polish MiG with a Ukrainian pilot somehow more provocative than a Turkish drone or an American antitank missile? Transferring planes isn’t the same as NATO aviators directly shooting down Russian jets.

Mr. Putin calls anything beyond Western acquiescence and Ukraine’s surrender a provocation. And NATO Secretary-General Jens Stoltenberg felt obliged to warn Mr. Putin Tuesday that a Russian attack on supply lines in alliance territory would trigger a collective response: “We are removing any room for miscalculation, misunderstanding about our commitment to defend every inch of NATO territory.”

Poland—which shares a border with Russia, Belarus and Ukraine—doesn’t want the transfer of planes directly to Ukraine from its territory to be perceived as a unilateral provocation. Polish Prime Minister Mateusz Morawiecki said Wednesday that the decision “must be unanimous and unequivocally taken by all of the North Atlantic Alliance.”

On Wednesday U.S. Defense Secretary Lloyd Austin formally nixed the MiG transfer to Ukraine. The failure of Team Biden to back up Warsaw is a failure of U.S. leadership.

There is risk of escalation in any war, and needless provocations should be avoided. But the risk of giving Mr. Putin a veto over NATO actions is that it undermines the credibility of deterrence. As Mr. Putin’s frustration grows, he is bombing cities, and Wednesday bombed a maternity hospital. The death toll is rising.

As he escalates, will he use chemical weapons or tactical nukes? Will NATO refuse to respond then because it fears World War III? The MiG mistake may let Mr. Putin believe his threats will make NATO stand down.