naomi; Let's start with the country's history, - cut and paste warning! "The region between the Tigris and Euphrates rivers is often referred to as Mesopotamia and thought to be the birthplace of writing and the world's oldest civilizations. The area has been home to continuous successive civilizations since the 6th millennium BC. At different periods in its history, Iraq was the center of the indigenous Akkadian, Sumerian, Assyrian, and Babylonian empires. It was also part of the Median, Achaemenid, Hellenistic, Parthian, Sassanid, Roman, Rashidun, Umayyad, Abbasid, Mongol, Safavid, Afsharid, and Ottoman empires, and under British control as a League of Nations mandate.
Iraq's modern borders were mostly demarcated in 1920 by the League of Nations when the Ottoman Empire was divided by the Treaty of Sèvres. Iraq was placed under the authority of the United Kingdom as the British Mandate of Mesopotamia. A monarchy was established in 1921 and the Kingdom of Iraq gained independence from Britain in 1932. In 1958, the monarchy was overthrown and the Republic of Iraq was created. Iraq was controlled by the Arab Socialist Ba'ath Party from 1968 until 2003. After an invasion by the United States of America and its allies, Saddam Hussein's Ba'ath Party was removed from power and multi-party parliamentary elections were held. The American presence in Iraq ended in 2011 but the Iraqi insurgency continued and intensified as fighters from the Syrian Civil War spilled into the country."
Then geography, I can't do that here, so take a look at a map of the middle east and ask why there are so many straight-line borders bearing little or no relationship to ethnicity, tribes, religions and their different factions races and creeds, languages and history.