Biden endorses Finland and Sweden’s bids to join NATO.

President Biden formally endorsed Finland and Sweden’s applications to join NATO on Wednesday, while also issuing a carefully-worded warning to Russia that the U.S. would help defend the countries in the interim before they become members of the alliance covered by its commitment that “an attack on one is as attack on all.”

“While their applications for NATO membership are being considered,” Mr. Biden said, “the United States will work with Finland and Sweden to remain vigilant against any threats to our shared security and to deter and confront aggression or the threat of aggression.”

Mr. Biden’s pledge is far short of a treaty, or even an executive agreement committing the United States to come to the aid of both countries. Instead, it amounts to what in the national security world is known as a “declaratory policy” — a declaration of the President’s intent.

There are other examples of such policies: Several presidents, for example, have vowed — through similar declarations — to make North Korea pay a price if it were ever caught exporting nuclear fuel to a foreign power.

Finland and Sweden formally asked to join the military alliance earlier on Wednesday, but they need support of all 30 nations in the group, a process that can require diplomatic bargaining. Later on Wednesday, Turkey blocked an initial effort by the alliance to move ahead quickly with their applications, according to a senior diplomat.

Mr. Biden timed the statement to precede a visit to the White House on Thursday by Prime Minister Magdalena Andersson of Sweden and President Sauli Niinisto of Finland. But the statement was negotiated weeks ago by U.S. defense officials as they worked with the countries on a pathway to becoming the 31st and 32nd members of NATO.

Should Sweden and Finland be accepted as part of the alliance, it would expand NATO at a time when President Vladimir V. Putin of Russia has vowed to push it back from his country’s borders and limit its eastward expansion. Finland shares an 830-mile frontier with Russia.

Jake Sullivan, the president’s national security adviser, told reporters on Wednesday that “we will not tolerate any aggression against Finland or Sweden during this process.”

But he offered few specifics, saying that “those are ongoing conversations that are happening at an operational and technical level between our Department of Defense and their ministries of defense, and also with other NATO allies and partners.”