The United States and its allies have sharply increased their airstrikes against the sprawling oil fields that the Islamic State controls in eastern Syria in an effort to disrupt one of the terrorist group’s main sources of revenue, American officials said this week. For months, the United States has been frustrated by the Islamic State’s ability to keep producing and exporting oil — what Defense Secretary Ashton B. Carter recently called “a critical pillar of the financial infrastructure” of the group — which generates about $40 million a month, or nearly $500 million a year, according to Treasury Department estimates. While the American-led air campaign has conducted periodic airstrikes against oil refineries and other production facilities in eastern Syria that the […]