Thanks to multiple accidents of history -- the uneasy presence of Sunni and Shiite Arab minorities ; an embittered local ethnic group , the Turkmens ; meddlesome neighboring countries with their own restive Kurdish populations ; and , not least , control of about 40 percent of Iraq 's known oil reserves -- the city of Kirkuk , population about 850,000 , is where all the pieces of Iraqi politics come together , or where they may well fall apart .