The flow of foreign fighters to the ranks of the Islamic State — once a mighty current of thousands of radicalized men and women converging on Syrian and Iraqi battlefields from nations across the globe — has been cut to a trickle this year as the group’s territory has shrunk and its ambitions have withered.

The decline, officials and experts say, has been dramatic, prolonged and geographically widespread, with the number of Europeans, Americans, North Africans and others joining up to fight and die for the idea of a revived Islamic caliphate falling as precipitously as the terrorist group’s fortunes.

From a peak of 2,000 foreign recruits crossing the Turkey-Syria border each month, the Islamic State and other extremist groups operating in Syria are down to as few as 50, according to U.S. intelligence assessments.  […]