Zelensky’s ambitions were clear: He would demand nothing less than full NATO membership for Ukraine.

At the July 2022 summit, his requests were less vociferous. Zelensky only launched his bid for fast-track membership to join the alliance in September, after Russian President Vladimir Putin announced that he would recognize four Ukrainian regions – Luhansk, Donetsk, Kherson and Zaporizhzhia – as Russian territory.

Swiftly after Putin’s claims, Zelensky said that Ukraine was applying for NATO membership “under an accelerated procedure,” meaning that the Vilnius summit is the first time the issue has been on the alliance’s agenda.

Under NATO’s open-door policy, any European country is free to apply, which is why its numbers have swelled from 12 founding members to the 31 it has today – soon to be made 32 by Sweden’s accession.

The key section of the treaty is Article 5. Its commitment clause defines the casus foederis. It commits each member state to consider an armed attack against one member state, in the areas defined by Article 6, to be an armed attack against them all. Upon such attack, each member state is to assist by taking "such action as [the member state] deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area." The article has only been invoked once, but considered in a number of other cases.

Zelensky left the summit without a timeline — but he did receive pledges for long-term security assistance by the United States and other G-7 nations. Biden has likened the aid to the kind that the United States provides Israel, and told reporters traveling with him that Zelensky was pleased.

"The one thing Zelensky understands now is that whether or not he's in NATO now is not relevant, as long as he has the commitments," Biden said, shortly before leaving Vilnius for Helsinki.