DULUIYAH, Iraq (AP) — The consequences of the al-Jabouri tribe's decision to ally with Iraq's Shiite-led government against their fellow Sunnis in the Islamic State group are etched on row after row of gravestones in this palm-shaded town by the Tigris River. Sunni tribes played a key role in driving out al-Qaida in Iraq — a precursor to the IS group — and are widely seen as the only force capable of securing the country's northwest Sunni heartland. [...] the few Sunni tribes that have stood up to the IS group have paid a heavy price, and anger at the Shiite-led government runs deep in the areas of northern and western Iraq that now make up the extremist group's self-styled caliphate. To do that, the government will have to somehow reverse the centrifugal forces unleashed by the 2003 U.S.-led invasion, which toppled Sunni dictator Saddam Hussein and handed power to the long-oppressed Shiite majority. In November, the extremists killed more than 200 men, women and children from the Sunni Al Bu Nimr tribe in the western Anbar province, apparently viewing it as a threat.