STORY: Finland and Sweden formally applied to join the NATO alliance on Wednesday (18 May), a decision spurred by Russia's invasion of Ukraine.
Accession is expected to take only a few weeks.
But they face objections from Turkey.
Neutral throughout the Cold War, Sweden and Finland's decision to join NATO is one of the most significant changes in Europe's security set-up for decades.
It reflects a sweeping shift of public opinion in the Nordic region since Russia's invasion.
During a short ceremony at allied headquarters, in which the Swedish and Finnish ambassadors to the alliance handed over their application letters, NATO Secretary-General Jens Stoltenberg welcomed the move:
"And I warmly welcome the requests by Finland and Sweden to join NATO. You are our closest partners, and your membership in NATO would increase our shared security. The applications you have made today are an historic step. (NATO) allies will now consider the next steps on the path to NATO."
The Nordic countries and their many backers now face uncertain months. All 30 of NATO's members need to approve their membership.
Ratification by all allied parliaments could take up to a year, diplomats say.
Turkey has surprised its allies in recent days by saying it had reservations about Finnish and Swedish membership.
It said the two countries harbor individuals linked to groups it deems terrorists and hit out at arms export embargoes imposed on it after its Syria incursion in 2019.
This was Turkish President Tayyip Erdogan speaking in Parliament on Wednesday:
"We expect our allies to understand our sensitivities, then show respect, and ultimately support where possible. Those sensitivities that we have, is to protect our borders against attacks by terrorist organizations. For years, we suffered a lot because of this, we lost a lot, we paid a heavy price and we are still paying."
Stoltenberg said on Wednesday he thought the issues could be resolved.
"The security interests of all allies have to be taken into account, and we are determined to work through all issues and reach rapid conclusions."
Their decision to gather under the NATO umbrella represents a setback for Moscow, with the war in Ukraine triggering the enlargement of the alliance on Russia's borders that its invasion was supposed to prevent.