“Zelensky is always asking for the sky and that’s perfectly appropriate, and it’s our job to make sure his job is nimble enough to meet the moment,” Mr. Murphy said. “We also do have an obligation to the taxpayers to not waste money.”
With each new request from Ukraine for another advanced capability, the United States has tried to assess how Mr. Putin might react by looking at the Kremlin’s comments and at how Russia has responded in the past when the United States has aided its allies and partners in Europe.
One thing above all others has influenced the debate within the administration over what weapons system to give Ukraine: Russia’s restraint in keeping the war contained.
Russia has steadily increased the brutality and breadth of its attacks against Ukraine, killing civilians on the march to Kyiv, the capital, deporting children from occupied areas and now trying to break the will of the Ukrainians by attacking the electrical infrastructure to plunge the country into cold and darkness.
But Moscow so far has not let its war spill over into NATO territory. American officials continue to insist they have seen nothing that indicates Russia has decided to expand its attacks beyond Ukraine.
There have been no stepped-up cyberattacks by Russian intelligence agencies on NATO allies, and no evidence that Russia has conducted any sabotage attacks on allied countries.
Mr. Putin’s unwillingness to fight NATO directly has been key to the alliance’s ability to supply Ukraine with a steady flow of arms and ammunition, the very supplies that have kept Kyiv in the fight. Mr. Putin has shown he will accept high levels of international support for Ukraine, as long as those weapons are used in Ukraine. That, U.S. officials said, is the critical calculus: whether Mr. Putin will see a weapons system as something meant to attack Moscow, or something meant to be used inside Ukraine.
It is important, these U.S. officials say, not to give Mr. Putin an excuse to expand the war.
Edward Wong contributed reporting.