Oil smuggling has been so enormously profitable that no matter how many obstacles Washington and Brussels put up, the barrels kept flowing. With a daily turnover of $1 billion, the black market has been just too attractive. For the first time, however, I see cracks in the illicit business. Millions of barrels of unsold Iranian and Russian crude are accumulating in storage. The reason isn’t just more U.S. and European sanctions and political pressure. Sure, they’ve helped. But the key factor is more mundane: The buyers of sanctioned crude oil have plenty of alternative aboveboard barrels available — at reasonable...