Finland and Sweden formally applied to join the Nato alliance on Wednesday at allied headquarters, a decision spurred by Russia's invasion of Ukraine, but face objections from Turkey to an accession process that is expected to take only a few weeks.

Neutral throughout the Cold War, Sweden's and Finland's decision to join Nato is one of the most significant changes in Europe's security architecture for decades, reflecting a sweeping shift in public opinion in the Nordic region since Russia's Feb. 24 invasion.

"This is a historic moment, which we must seize," Nato Secretary-General Jens Stoltenberg said at a short ceremony in which the Swedish and Finnish ambassadors to the alliance handed over their application letters, each in a white folder embossed with their national flag.

"I warmly welcome...