Finland and Sweden on Wednesday applied to join NATO, in a move that would bolster the military alliance’s power against Russia as Moscow continues its war in Ukraine.

So, what’s next? NATO officials have said the membership application could be completed “in a couple of weeks” — but becoming full-fledged members will take longer. Here’s a brief guide on what to know about expanding the world’s biggest security alliance, formally named the North Atlantic Treaty Organization.

Must all current NATO members agree to admit a new one?

Yes. As explained on the alliance’s web site, a decision on inviting a country to join is taken by the North Atlantic Council — NATO’s main political decision-making body — “on the basis of consensus among all Allies.”

Finland and Sweden could become members within a matter of months if accession talks go well. The process usually takes eight to 12 months, as the Associated Press has reported, but NATO wants to move quickly given the threat from Russia.

NATO Secretary-General Jens Stoltenberg said Wednesday that the allies “are determined to work through all issues and reach rapid conclusions.” NATO now has 30 members, with the newest being North Macedonia.

Read: Finland, Sweden officially apply to join NATO, driven by security concerns over Russia’s invasion of Ukraine

Is Turkey an obstacle?

A stumbling block to admitting the two countries could be Turkey. President Recep Tayyip Erdogan has said he’s opposed to the two countries joining, citing the presence of alleged Kurdish militants in Sweden and Finland.

Earlier this week, meanwhile, U.S. Secretary of State Antony Blinken said he was confident that members would reach consensus on the two new applicants.

On Monday, Stoltenberg called Turkey a “valued ally” and said in a tweet that “any security concerns need to be addressed. We must stand together at this historic moment.”

What does Russia think of Finland and Sweden joining?

Russian President Vladimir Putin said Monday he did not have an issue with the two new applicants. But that came with a caveat: he doesn’t want a military buildup in either country.

“The expansion of military infrastructure into this territory will certainly provoke our response,” Putin said in televised comments.

What’s the U.S. role?

President Joe Biden is scheduled to host Prime Minister Magdalena Andersson of Sweden and President Sauli Niinistö of Finland at the White House for a meeting Thursday.

“We welcome the recent statements from governments of Finland and Sweden on their intention to seek NATO membership,” White House press secretary Karine Jean-Pierre said Monday. “We will strongly support these applications when they are formally presented in Brussels.  Both Finland and Sweden are close and valued defense partners of the United States and of NATO,” she said.

Now see: Biden to host leaders of Sweden and Finland amid NATO bids