President Biden welcomed Prime Minister Ulf Kristersson of Sweden at the White House today to emphasize America’s support for the Nordic nation’s swift acceptance into NATO.

Sweden’s entry into NATO would be a significant blow to President Vladimir Putin of Russia, who has sought to halt the alliance’s expansion. Sweden broke from decades of neutrality when it asked to join after Russia’s invasion of Ukraine last year.

But as the alliance prepares for a show of unity at a summit in Lithuania next week, the only major barrier is President Recep Tayyip Erdogan of Turkey, a NATO ally. He has been able to block Sweden because acceptance requires unanimous consent.

Erdogan’s objection centers on his accusation that Sweden has harbored Kurdish exiles and refugees associated with what Turkey considers a terrorist group. The U.S. has tried to placate the Turkish leader, including by supporting the sale of new F-16 fighter jets to Turkey, but Erdogan hasn’t budged. (The burning of a Quran in Stockholm last week could further complicate matters.)