BEIRUT, Lebanon — Struggling to keep Iraq from splintering, American diplomats pushed for a law in 2011 to share the country’s oil wealth among its fractious regions.

Then Exxon Mobil showed up.

Under its chief executive, Rex W. Tillerson, the giant oil company sidestepped Baghdad and Washington, signing a deal directly with the Kurdish administration in the country’s north. The move undermined Iraq’s central government, strengthened Kurdish independence ambitions and contravened the stated goals of the United States.

Mr. Tillerson’s willingness to cut a deal regardless of the political consequences speaks volumes about Exxon Mobil’s influence. In the Iraq case, Mr. Tillerson and his company outmaneuvered the State Department, which he has now been nominated by President-elect Donald J. Trump to lead.

“They are very powerful in the region, and they couldn’t care less about what the State Department wants to do,” Jean-François Seznec, a senior fellow at the Atlantic Council, a research group in Washington, said of Exxon Mobil’s pursuits in the Middle East.