President Joe Biden formally welcomed Finland and Sweden joining the NATO alliance Tuesday as he signed the instruments of ratification that delivered the US’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 US 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 to the alliance, which is expected in the coming months.