A Slight Miscalculation

These airstrikes were launched not because U.S. officials were prescient. They came after the Obama administration found and quietly fixed a colossal miscalculation. U.S. intelligence had grossly overestimated the damage they’d inflicted during airstrikes on the militants’ oil production apparatus last year, while underestimating Islamic State’s oil revenue by $400 million.

That's four times as much as the administration had previously believed they were getting, and doesn't count income from the slave trade, general crime and extortion in its area of operation, etc.