New U.S. intelligence assessment casts doubts on Bush's Iraq policy
From the above report the following defines zero-sum politics based on sectarian and tribal forces:

The new intelligence findings were contained in a 23-page Global Security Assessment presented to the House Armed Services Committee by Thomas Fingar, the chairman of the National Intelligence Council, the intelligence community's top analytical body.

"The struggle among and within Iraqi communities over national identity and the distribution of power has eclipsed attacks by Iraqis against (U.S.-led) Coalition Forces as the greatest impediment to Iraq's future as a peaceful, democratic and unified state," said the report by the Office of the Director of National Intelligence.
A former senior military official who advises the Pentagon said there is mounting concern that hard-line Sunni and Shiite leaders, believing their side can prevail over the other in an all-out conflict, do not intend to implement the benchmarks so they can hasten a U.S. troop withdrawal.

"Both sides believe there is no point in having part of the pie if they can have the whole pie, and they are both convinced they can overwhelm their opponent," said the official, who requested anonymity to protect his relationship with the Pentagon.