WARSAW — If Russian President Vladimir V. Putin were to order tanks into other European countries, the nine nations along NATO’s eastern flank would be the likeliest targets.
So on Wednesday, President Biden will meet with the leaders of those countries, offering reassurance that the United States is prepared to defend the most vulnerable members of the alliance from the kind of assault now taking place in Ukraine.
The meeting, slated to take place just hours before Mr. Biden concludes his three-day visit to Ukraine and Poland, is meant to underscore his administration’s determination to confront Mr. Putin and prevent him from intimidating some of the world’s more fragile democracies.
“These are largely the group of eastern flank NATO allies who are basically and, quite frankly, literally on the front lines of our collective defense right now,” John Kirby, a senior spokesman for the National Security Council, told reporters before the president’s trip.
Mr. Kirby said the president would meet with leaders of the nine countries — Bulgaria, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania and Slovakia — to “reaffirm the United States’ unwavering support for the security of that alliance and trans-Atlantic unity.”
In his speech from the royal castle in Warsaw on Tuesday, Mr. Biden was resolute about the American commitment to the defense of all of the North Atlantic Treaty Organization, declaring that the guarantee to come to the aid of any threatened ally “is rock solid. And every member of NATO knows it. And Russia knows it as well.”
But the threat of direct military intervention is less theoretical for Poland, Hungary and the other countries whose borders lie not far from Russian territory than for Britain, France or Spain.
For those countries, Mr. Putin’s invasion of Ukraine almost a year ago is not only a threat to the principle of democracy, but also a road map for an invasion that could just as quickly plunge their own cities into the darkness of war.
Recognizing that special threat, the leaders of the nine countries came together in 2015 to discuss the steps they could take to reduce the risk. The gathering on Wednesday with the American president will be the most high-profile, and urgent, meeting since the group was formed eight years ago.
For Mr. Biden, it will be another opportunity to praise NATO’s unity in the face of Mr. Putin’s actions. In his briefing last week, Mr. Kirby said that the group would focus on maintaining a unity of purpose even as the war in Ukraine drags on.
White House officials said the United States and its allies would soon announce tougher sanctions on Russia aimed at increasing the cost of Mr. Putin’s war.
“The leaders will discuss our efforts over the past year to strengthen NATO, which is stronger and now more united than it was — than it has ever been, and how each of our nations will continue to work together,” he said.
After the meeting, Mr. Biden is expected to depart Poland and return to Washington, concluding a foreign trip that began with his top secret visit to the capital of Ukraine.