NATO invited Sweden and Finland to join the military alliance on Wednesday in one of the biggest shifts in European security in decades.
The traditionally neutral Nordic countries made their bids to join following Russia's invasion of Ukraine.
NATO also agreed formally treat Russia as the "most significant and direct threat to the allies' security," according to a summit statement.
President Joe Biden said the U.S-led alliance would be ready to deal with threats from all directions.
He also said the U.S. would ramp up forces in Europe, in response to threats from Russia.
Biden's pledge came as Ukrainian President Volodymyr Zelenskiy appeared at the summit virtually to appeal for more weapons.
The bloc's 30 allies took the decision to admit Sweden and Finland at their summit in Madrid.
Ratification in allied parliaments is likely to take up to a year.
But once it is done, Finland and Sweden will be covered by NATO's Article 5 collective defense clause.
Initially Turkey vetoed the Nordic countries' bids to join, due to concerns about terrorism.
But President Tayyip Erdogan gave them the green light on Tuesday evening, after agreeing to a series of security measures with his Finnish and Swedish counterparts.
The North Atlantic Treaty Organization was founded in 1949 to defend against the Soviet threat.