Sweden and Finland have handed in their formal applications to join NATO.
On Wednesday morning local time, the two countries' envoys delivered their applications to NATO chief Jens Stoltenberg.
"The applications you have made today are an historic step. Allies will now consider the next steps on your path to NATO,"
The two Nordic countries have been neutral for decades.
But they've changed their stance because of the threat to European security posed by Russia's invasion of Ukraine.
Public opinion in both countries has swung heavily in favor now of breaking with neutrality and fortifying Europe's defenses.
"It's a message of strength and a clear signal that we stand united going into the future ahead."
If both countries are admitted, it would mark the biggest NATO expansion in decades, increasing its membership to 32 countries.
Finland's border with Russia, 13-hundred kilometers long, would be the longest single land border between Russia and the NATO allies.
The process of joining NATO can take up to a year because of the need for ratification by all 30 of NATO's existing members.
But Sweden and Finland could face more obstacles with Turkey recently expressing a reluctance to admit them due to what it says is their refusal to deal with Kurdish militants.
But Secretary General Stoltenberg made clear on Wednesday that NATO will move to resolve all outstanding issues and reach a conclusion quickly.
Kim Yeon-seung, Arirang News