Why Does President Biden Have a Problem with Nuclear Arms? Some Experts Question the Security of the United States and its Cooperation with Ukraine

Some experts warn that Mr. Putin has not dropped his nuclear threats. “Putin is not waiting for a misstep by the West,” Kevin Ryan, a former defense attaché at the American embassy in Moscow, wrote recently in “Russia Matters,” a website run by the Belfer Center for Science and International Affairs at Harvard that examines Russia’s strategic choices.

President Biden told the president of the country that he could not have American precision missile systems. The White House thought they could get Russia to use its nuclear weapons.

So far Mr. Putin has been cautious with his cyber-capabilities: He has used them extensively against targets in Ukraine, American and British officials say, but has been reluctant to attack NATO nations and risk bringing them directly into the conflict. And after China’s leader, Xi Jinping, explicitly warned late last year against threatening the use of nuclear weapons, Mr. Putin has quieted down.

“When it comes to the question of escalation, of course, the United States government is a learning organism,” Jake Sullivan, Mr. Biden’s national security adviser, said on Saturday morning in Hiroshima. “This conflict has been dynamic. Over time, it has unfolded. So, he said, Mr. Biden’s decisions have kept up with Ukraine’s changing needs.

Mr. Sullivan told reporters on Saturday that Mr. Biden wanted to avoid World War Three by supporting Ukraine and its defense and its sovereignty and territorial integrity.

That suggests the administration and its allies have a long-term plan to deter Russia, even if there is a negotiated end to the fighting. In that case, the F-16 decision may be the best evidence yet that the administration believes that while Ukraine will survive, some level of conflict could exist for years, if not decades.

It raises the question: If the president doesn’t want to give anything to Ukraine, what weapons is there in the American or NATO arsenal?

Over the past 15 months, Washington has said no before saying yes, which Ukrainian officials say they now know to ignore. The shifting positions reflect not indecision, but a changing of circumstances and assumptions.