Ukraine has a relatively small number — and it's not saying how many. Russia has apparently taken some of them out — and desperately want to eliminate all of them, but it hasn't been able to do so. "Those are incredibly important to Ukraine," said Obrien. "They don't threaten every Russian flight. They can't threaten every flight. But they can make the pilots jumpy enough that anywhere they want to fly there's a potential threat."

Yes, but their air defenses were relatively limited, especially when the war began. The Americans have helped with large numbers of Stinger missiles. A single soldier fires these missiles from his shoulder, and they're very effective at taking down low-flying helicopters. But perhaps Ukraine's most under-appreciated weapon in this war is the S-300 surface-to-air missile system. This is a hulking, Soviet-era air defense system that fires missiles from the ground that take down jet fighters.

It is possible that Russia will try to “break Ukraine into parts and leave Western Ukraine alone,” said Angela Stent, a specialist in Russia affairs at Georgetown University. But because that would leave a West-leaning government in Kyiv that Mr. Putin has previously depicted as illegitimate, she said, “I find it hard to imagine that.”More likely, analysts said, is for Russia, at a minimum, to seek a constitution of Ukraine that grants significant independence to the east of the country, and an effective veto over Ukrainian government action.The Ukrainians could agree to further elections—but the risk for Russia is that, even in the east, it wouldn’t like the outcome.Mr. Clarke said one model of Ukrainian neutrality that might appeal to Russia is that of Austria in 1955. The Soviets pulled out of Austria in return for a constitutional guarantee of neutrality that exists today.Rather than persuade Ukrainians that neutrality is an attractive option, the invasion is likely to harden opinion in the opposite direction.

Those sanctions can be ratcheted up or down depending on Russian actions. They aren’t without cost for the West. They are likely to intensify an existing inflation problem and, if Russia retaliates by cutting off energy supplies, could lead to electricity rationing in Europe.Mr. Sherr of the Estonian Foreign Policy Institute doesn’t expect those measures to change Mr. Putin’s mind. “Putin and the people around him, at least the political and security people around him, have never bowed to the logic of sanctions,” he said.

Russia’s air force, navy and nuclear force have been partly or completely modernized, he said, but the army looks as though it hasn’t overcome past weaknesses.

“We’ve all been astonished that this new Russian army looks like the old Red Army—not very well trained, not very well commanded, with really quite poor logistics—which implies either a big failure of planning or…a big underestimation of the enemy,” said Mr. Clarke, the former director of the Royal United Services Institute.

The full scale of this Russian setback is emerging only now, with satellite imagery showing more than 70 Russian tanks, armored personnel carriers and other armor destroyed after Ukrainian artillery and airstrikes sank three pontoon bridges and shelled the Russian beachhead in Bilohorivka.“We have never seen such dumb stubbornness, going with a frontal assault and trying to build pontoons in the same place three times in a row. But they still keep trying,” said Luhansk Gov. Serhiy Haidai. He added that Ukrainian artillery keeps shelling the area and, according to intelligence intercepts, an entire Russian battalion is refusing orders to attempt yet another crossing in Bilohorivka. That claim couldn’t be independently confirmed.