It’s been a month since President Obama announced that the U.S. would engage in a sustained campaign of airstrikes against Islamic State, the militant Sunni rebellion in Syria and Iraq. The idea was to bomb Islamic State into nonexistence, which has proved difficult. Not only is the movement well-armed, battle-hardened, and deeply entrenched in much of Syria and northern Iraq, it’s also very well-financed, thanks to oil wells and refineries it’s been able to capture. By late June, Islamic State was raising as much as $2 million a day refining and smuggling oil, making it one of history’s wealthiest terrorist groups. Though the airstrikes have failed to keep Islamic State from advancing in the field, they have apparently succeeded in dismantling its sophisticated oil network, reducing the movement’s ability to make gasoline and diesel for its tanks and trucks and cutting off a vital source of funding. […]