Initially of the struggle in Ukraine, President Biden advised the nation’s president, Volodymyr Zelensky, that he couldn’t have American precision missile techniques. The White Home feared they might tip Russia into reaching for its tactical nuclear weapons.

Then he allowed them.

The identical dynamic infused debates over offering tanks a number of months in the past. Now Mr. Biden, who in February rejected F-16 fighter jets as pointless, met in Hiroshima on Friday with leaders of different main democracies and advised them that he would enable Ukrainian pilots to be skilled on the American-made warplanes. He added that in a number of months, the allies would work out easy methods to start delivering fashionable Western fighters to a Ukrainian pressure struggling to maintain an getting old, dwindling fleet of pieced-together, Soviet-made fighters within the air.

All of it raises the query: Are there any typical weapons within the American or NATO arsenals that the president wouldn’t, ultimately, present to Ukraine?

Washington’s sample of claiming no earlier than saying sure has repeated itself sufficient instances over the previous 15 months that Ukrainian officers say they now know to disregard the primary reply and hold urgent. However White Home officers say the shifting positions mirror not indecision, however altering circumstances — and altering assumptions in regards to the dangers concerned.

“In relation to the query of escalation, after all, the US authorities is a studying organism,” Jake Sullivan, Mr. Biden’s nationwide safety adviser, mentioned on Saturday morning in Hiroshima. “This battle has been dynamic. It has unfolded over time.” So, he mentioned, Mr. Biden’s selections have saved up with Ukraine’s altering wants.

Within the weeks after the invasion, the teetering Ukrainian authorities wanted Stinger missiles and different anti-tank techniques. When the struggle shifted to the south and the east of the nation, with large open plains, they wanted artillery and air defenses — and 155-millimeter howitzer shells. And whereas Mr. Biden doesn’t imagine fighter jets will play an vital position within the battle for some time, offering them is a part of serious about easy methods to defend Ukraine for the long run — after the present section of the struggle is over.

That means the administration and its allies now imagine that even when there’s a negotiated finish to the combating — maybe a Korea-like armistice — Ukraine will want a long-term functionality to discourage an offended, sanctioned Russia. In that case, the F-16 determination could also be the most effective proof but that the administration believes that whereas Ukraine will survive, some degree of battle might exist for years, if not many years.

In a briefing to reporters on Saturday, Mr. Sullivan repeated Mr. Biden’s two touchstones: “Assist Ukraine and its protection and its sovereignty and territorial integrity” whereas continuing “in a approach that avoids World Battle Three.”

The latter is a phrase Mr. Biden has used typically together with his employees. However the considering behind what it means to keep away from World Battle III has advanced. Weapons that Washington thought would possibly set off escalation have turned out not to take action. As not too long ago as 5 months in the past, White Home officers have been nervous that Mr. Putin would conclude that his military will take a decade to rebuild after the catastrophe it has introduced on itself. That would go away him with solely two viable choices: utilizing his formidable cyberweapons to cripple infrastructure, or threatening to make use of his nuclear arsenal, in hopes of freezing Western assist to Ukraine.

To this point Mr. Putin has been cautious together with his cyber-capabilities: He has used them extensively towards targets in Ukraine, American and British officers say, however has been reluctant to assault NATO nations and danger bringing them straight into the battle. And after China’s chief, Xi Jinping, explicitly warned late final 12 months towards threatening the usage of nuclear weapons, Mr. Putin has quieted down.

However few assume that’s for lengthy. Nobody is aware of what would possibly set off Mr. Putin, although Russian officers have particularly warned towards giving Ukraine ATACMS, a long-range precision missile system made by Lockheed Martin that will allow Mr. Zelensky to focus on Crimea, and Russian bases there, from afar.

Some consultants warn that Mr. Putin hasn’t dropped his nuclear threats; simply delayed them. “Putin isn’t ready for a misstep by the West,” Kevin Ryan, a former protection attaché on the American embassy in Moscow, wrote not too long ago in “Russia Issues,” a web site run by the Belfer Middle for Science and Worldwide Affairs at Harvard that examines Russia’s strategic selections.

“He has been constructing the circumstances for nuclear use in Ukraine since early within the struggle and is able to use a nuclear weapon at any time when he decides, more than likely in response to his faltering navy’s incapability to escalate as a lot as he needs by typical means,” wrote Mr. Ryan, a retired brigadier basic, who directs back-channel talks with retired Russian navy officers.