Saudi Arabia has frustrated American policy makers for years. Ostensibly a critical ally, sheltered from its enemies by American arms and aid, the kingdom has spent untold millions promoting Wahhabism, the radical form of Sunni Islam that inspired the 9/11 hijackers and that now inflames the Islamic State.

The latest chapter in this long, sorrowful history involves tiny Kosovo. With a population of only 1.8 million people, Kosovo has sent more of its young people per capita than any other country to fight and die in Iraq and Syria. Since 2012, some 314 Kosovars have joined the Islamic State, including two suicide bombers, 44 women and 28 children. Even Belgium, widely seen as a hotbed of extremism after the attacks on Paris and Brussels, lags behind it in the recruitment rankings.

As detailed by Carlotta Gall in a recent article in The Times, Kosovo is in this position largely because Saudi Arabia and other Persian Gulf states have spent years developing and funding a network of imams, mosques and secretive associations there. And while there is no evidence that any group gave money directly and explicitly to persuade Kosovars to go to Syria, senior officials in Kosovo told Ms. Gall that extremist clerics and groups have spent heavily to promote radical Islamic thinking among young and vulnerable people. “The issue is they supported thinkers who promote violence and jihad in the name of protecting Islam,” Fatos Makolli, head of Kosovo’s counterterrorism police, told her.

The United States and NATO invested heavily in helping Kosovo gain independence from Serbia in 2008 and establish democracy. That Saudi Arabia should be using Kosovo as a breeding ground for extremists, or allowing it to be used as a breeding ground by any Saudi entity or citizen, is a cruel reminder of the contradictory and even duplicitous behavior of America’s partners in the Persian Gulf and helps to explain why its relationships with those countries have become increasingly troubled.