CHISINAU – Over the pulsating beat at an exclusive nightclub, the arms smuggler made his pitch to a client: €2.5 million for enough radioactive cesium to contaminate several city blocks. It was earlier this year, and the two men were plotting their deal at an unlikely spot: the terrace of Cocos Prive, a dance club and sushi bar in Chisinau, the capital of Moldova. “You can make a dirty bomb, which would be perfect for the Islamic State,” the smuggler said. “If you have a connection with them, the business will go smoothly.” But the smuggler, Valentin Grossu, was not...