President Biden pledged an “ironclad” commitment to America’s allies on Tuesday as he sought to reassure NATO members that the United States would not abandon its international obligations despite the threats of aggression by Russia and disruption by his all-but-certain fall election rival, former President Donald J. Trump.

Hosting the leaders of Poland at the White House, Mr. Biden made a show of defiance not only against President Vladimir V. Putin of Russia, who continues to wage war on Ukraine, but also, implicitly at least, against Mr. Trump, who has said he would “encourage” Russia to attack NATO members that do not spend enough on their militaries.

“When we stand together, no force on earth is more powerful,” Mr. Biden said, citing former Secretary of State Madeleine K. Albright, the Czech-born diplomat who championed NATO membership for Poland, Hungary and her homeland, finalized on this day in 1999 with the support of Mr. Biden, then a senator. “I believed that then, and I believe it now,” he added.

The meeting with President Andrzej Duda and Prime Minister Donald Tusk of Poland marked the 25th anniversary of their country’s accession to NATO, which was the beginning of a quarter-century post-Cold War expansion of the alliance into parts of Europe previously dominated by the old Soviet Union. Just this month, NATO formally welcomed Sweden, which like Finland joined in response to Russia’s full-fledged invasion of Ukraine in 2022, increasing the alliance to 32 members, up from 16 when the Berlin Wall fell.

Mr. Biden used the occasion to announce $300 million in additional weapons and military equipment for Ukraine, a small fraction of what he has asked Congress for but all he could squeeze out of savings from previous arms transfers without additional legislative authority. He pressed House Republican leaders to stop blocking legislation passed by the Senate for $60 billion in security aid to Ukraine, a measure that has bipartisan support and would almost surely pass if brought to the floor.

“We must act before it literally is too late, before it’s too late, because as Poland remembers, Russia won’t stop at Ukraine,” Mr. Biden said. “Putin will keep going, putting Europe, the United States and the entire free world at risk.”