WASHINGTON — President Joe Biden formally welcomed Finland and Sweden joining the NATO alliance Tuesday as he signed the instruments of ratification that delivered the U.S.’s formal backing of the Nordic nations entering the mutual defense pact, part of a reshaping of the European security posture after Russia’s invasion of Ukraine.

“In seeking to join NATO, Finland and Sweden are making a sacred commitment that an attack against one is an attack against all,” Biden said at the signing as he called the partnership the “indispensable alliance.”

The U.S. became the 23rd ally to approve NATO membership for the two countries. Biden said he spoke with the heads of both nations before signing the ratification and urged the remaining NATO members to finish their own ratification process “as quickly as possible.”

The Senate last week approved the two, once-non-aligned nations joining the alliance in a rare 95-1 vote that Biden said shows the world that “the United States of America can still do big things” with a sense of political unity.

The countries sought out NATO membership earlier this year to guarantee their security in the wake of Russian President Vladimir Putin’s offensive in Ukraine. The North Atlantic Treaty Organization’s rules require the consent of all of its 30 existing members before Finland and Sweden can officially accede into the alliance, which is expected in the coming months.

The candidacies of the two prosperous Northern European nations have won ratification from more than half of the NATO member nations in the roughly three months since the two applied. It marks one of the speediest expansions of the pact of mutual defense among the United States and democratic allies in Europe in its 73-year history.

U.S. State and Defense officials consider the two countries net “security providers,” strengthening NATO’s defense posture in the Baltics in particular. Finland is expected to exceed NATO’s 2% gross domestic product defense spending target in 2022, and Sweden has committed to meet the 2% goal.

Sweden and Finland applied to join NATO in May, setting aside their longstanding stance of military nonalignment. It was a major shift of security arrangements for the two countries after neighboring Russia launched its war on Ukraine in late February. Biden encouraged their joining and welcomed the two countries’ government heads to the White House in May, standing side by side with them in a display of U.S. backing.

The U.S. and its European allies have rallied with newfound partnership in the face of Putin’s military invasion, as well as the Russian leader’s sweeping statements this year condemning NATO, issuing veiled reminders of Russia’s nuclear arsenal and asserting Russia’s historical claims to territory of many of its neighbors.