General Milley also traveled to a training field near Nowa Deba, in southern Poland, where paratroopers from the 82nd Airborne have been training with Polish troops. The coordination is part of Mr. Biden’s effort to reassure Eastern European allies — and assure President Vladimir V. Putin of Russia — that while NATO is not sending troops into Ukraine, which is not a member of the alliance, it will fight any incursion into the territory of a NATO member.Officials were adamant that NATO was standing up to Mr. Putin. “NATO is more unified than I’ve ever seen NATO unified before,” Gen. Christopher G. Cavoli, the commander of U.S. Army Europe and Africa, told reporters at the training center. “I’ve been working since I was a second lieutenant, in 1988, and I can tell you I’ve never seen the resolve, and the practical expression of combat readiness, as I’ve seen in the alliance ground forces right now. It’s remarkable.”Shortly after meeting with troops at Nowa Deba, General Milley flew by helicopter to Rzeszow, Poland, the headquarters of the 82nd Airborne in the country, which, he said, was deployed to “deter any further territorial aggression by Russia.”Maj. Gen. Chris Donahue, who commanded the U.S. evacuation from Afghanistan after Kabul fell to the Taliban in August and who is the commanding general of the 82nd Airborne, said the Polish headquarters could temporarily house some 2,500 evacuees from Ukraine. The site, which is in an arena, is meant to serve as a staging ground for evacuees who are citizens of the United States and other NATO countries. Bunk beds filled the room, most of them empty.Officials said that while there had been a deluge of Ukrainian refugees crossing the border, the number of American evacuees was small. Most American citizens who left Ukraine traveled by air or crossed at other areas along Ukraine’s borders, the officials said.At an operations center in the arena, Polish border guards, American intelligence officials, paratroopers and State Department desk officers huddled over maps of Ukraine.“They were smart to bring in all U.S. government agencies and international partners working on solutions for Ukraine,” said Col. Dave Butler, a spokesman for General Milley.