Iraqi history is complex and fascinating and justice could never be done to in a single node. Here I will attempt to give an outline, chronologically, as well as highlight the most important themes. In the coming months, and possibly with your help, I will hopefully fill in in greater depth some of the more interesting concepts.
Beginnings ~ Ancient Civilisation is Born
Regardless of your beliefs about the origin of mankind and civilisation -- be they Darwinist or Creationist -- Iraq features at the very dawn of it all. Biblical Mesopotamia is traced to the alluvial plains between the Tigres and Euphrates rivers, also called the Fertile Crescent. The two rivers, in equal measures sustainers and destroyers of human life on a grand scale, cut Iraq into three more-or-less north/south slivers.
Settlement in Mesopotamia has been traced back to 6000 BC, when migrants from the Turkish and Iranian highlands discovered her fertile soils. Life in early Sumer was governed by the unpredictable rivers. The floods that deposited the fertile alluvium also swept away life, crops, shelters. On a balance, people prevailed, and for the first time ever, around 4000 BC, they were able to grow a surplus of food.
There's no point having more food than you can eat if you don't have a fridge. The natural progression was thus to an organised society, which in turn advanced agricultural practises and increased the surpluses. Word spread and more and more people moved to Sumer from further afield. Historic evidence suggests that the people of Sumer formed a cosmopolitan mix of linguistic and ethnic groups.
Sumerian society continued for thousands of years, during which time the Sumerians introduced the earth to writing; irrigation; the wheel; astronomy; the sixty-minute hour; the double-entry accounting practise; and literature. The first known recorded story is that of Gilgamesh, king of city-state Uruk around 2700 BC. The tale tells of his grief after his friend dies, which inspires him to pursue a quest for immortality. Another recurring story is one about a chap who survived a great flood by building an ark.
The first known religion actually predates the rise of Sumer: there is evidence of cult centres such as Eridu as far back as 5000 BC. People undertook pilgrimages to these cult centres, making them natural locations for the future growth and development of cities. Sumerians themselves were pantheistic, their gods reflecting the elements and challenges that made up their daily lives. Many of the ziggurats (temples, think Tower of Babel) that the Sumerians built of sunbaked brick still stand in Iraq today.
Sumerians also developed the wheeled chariot, created a step-change in warfare technology, discovered bronze and all the weaponry benefits it brought. Having weapons, as the Americans will tell you, is absolutely no fun at all unless you've got someone to use them on, so the Sumerians divided themselves up into kingships, each with their own city-state, and began to beat the crap out of each other. This is the general state-of-play for a couple thousand years, before the rest of the world caught up and stumbled upon the Fertile Crescent.
I'm cutting things pretty short here, and you'll want to look out for the following places:
Babylon
Babylon (from Bab-ili, meaning Gate of God) deserves a section of its own, if only because its the only place I've mentioned so far that you know anything about. Babylon first put itself on the world map around 1792 BC, under its sixth ruler, King Hammurabi. This chap ruled most of the Tigris-Euphrates river valley from the Persian (or Arabian, depending on your sympathies) Gulf to Assyria in the north. Hammurabi developed an administrative structure and introduced a code of law. The Hammurabi Code was not the first code of law in the world, but it was the most sophisticated. It was designed to cause justice to prevail in the country, to destroy the wicked and the evil, that the strong may not oppress the weak. Apart from such lovely ideals, he also included some practical laws about land tenure; rent; the position of women; marriage; divorce; inheritance; contracts; control of public order; administration of justice; wages and labour conditions. He invented the notion of the punishment must befit the crime, but you are probably more familiar with it in its "an eye for an eye" guise.
Into the second paragraph on Babylon and I still haven't mentioned the Hanging Gardens. Before getting onto Nebuchadnezzar and his Wonder of the World, we have a couple hundred years of argy-bargy. The Assyrians rose to prominence, went off on a regional pillaging tour, occupying Phoenician cities and conquering Damascus and Babylon and making Judah a vassal state. Sennacherib was not content with the cities he'd taken over, so he built himself a new capital for his new empire, Nineveh, and destroyed Babylon because its people had risen in revolt.
The Assyrians invented forced resettlement of their subject people, which did not win them any friends. In 612 BC, the revolters allied themselves with the forces of two new kingdoms: Medes and the Chaldeans (or Neo-Babylonians). They managed to extinguish Assyrian power, destroyed Nineveh and took over the Assyrian lands of Syria and Palestine. King Nebuchadnezzar took over in 605 BC and conquered the kingdom of Judah and destroyed Jerusalem in 586 BC. It was actually his descendent, Nebuchadnezzar II, who build the majesty of the Hanging Gardens of Babylon, reportedly for his home-sick wife.
Greeks and Iranians Enter the Fray
You'll probably be pleased to hear that Nebuchadnezzar and his clan did not last long. First came the Archaemenid Iranians, when Cyrus the Great incorporated Babylon into the Iranian empire and released the Jews who had been held in captivity there. You may think this feat makes him a nice guy, but let me tell you those Old Testament rulers were not a friendly bunch. Cyrus may have respected local customs, but he also wanted his dues: they had to pay a tribute and be particularly well behaved. These were the Persian Empire years, and Cyrus the Great was followed by Darius the Great who in turn was followed by Alexander the Great. (See a pattern developing? Except Alex was a Greek and I should have waited before mentioning him.)
Unfortunately for the Babylonians, the Iranian trade routes didn't go through their turf, so they fell into economic decline and impoverishment. The Iranians may not have spread their boodle, but they did spread their language. The linguistic complexity of Babylonia, or erstwhile Sumeria, became predominantly Aramaic speaking. Clearly the Iranians were not a popular bunch, so when Alexander the Great arrived, he was viewed as a liberator. Other nodes can tell you about him in much better detail, but I will mention that before he died in Babylon, he did restore trade to the area.
In 126 BC, the Greeks were ushered on their way by the Parthians (or Arsacids). They were "an intelligent, nomadic people" who had migrated from the Steppes of Turkistan to north-eastern Iran and conquered the Iranians while tending their goats. Under their tenure, Arabs, Iranians and Aramaeans joined the melting pot. Parthian/Arsacidian rule came to an abrupt end with the resurgence of an Iranian people, the Sassanids, who took back control in 227 AD.
The Sassanids were actually relative heathens and allowed Mesopotamia to fall to ruins. The canals and irrigation ditches had fallen into disrepair beyond use, and the rivers again took command. Booting the Sassanids out was childs play for the recently grouped Arabs.
Islam and the Arab Conquest
Brevity is not my strong point and I'll thank you to stay awake. It gets damn interesting here, and I also tell you about a pretty amazing woman so take notes.
The Arabs were known to the Iranians as a nice little tribal people more interested in their own paternalistic society and infighting than bothering the neighbours. Unfortunately for the unsuspecting Iranians, God had other plans. Muhammad, a Hashemite clansman from the powerful Quraysh tribe of Mecca, received divine inspiration that became what is still the world's fastest-growing religion. In spreading his monotheistic faith, Islam, Muhammad managed to unite the Arabian people.
Muhammad died in 632 AD, and among his widows was 19 year old Ayesha. Her father, Abu Bakr, and Muhammad had been tight most of her life, and when her husband died, her father became the first caliph. Not content with his spiritual role, Abu Bakr decided to take on both the Byzantine Empire and the Sassanid Empire.
Now don't you worry, I'm sure he was on something of a crusade (actually the original Jihad), but he got together an army of 18,000 tribesmen, appointed the brilliant general Khalid ibn al Walid (The Sword of Islam) and sent them off to the Euphrates delta. I don't know if it was luck or foresight, but handily the much larger Sassanid force had a spot of burn-out thanks to their own campaigns against the Byzantines. It must have helped that the Byzantine and the Sassanid Empires were culturally and socially bankrupt. In what became known as the Battle of the Chains (because defeated soldiers were chained together so they could not escape), Khalid and co emerged victorious.
Khalid did not appear to set out to win friends or influence people. He issued the locals with an ultimatum:
Accept the faith and you are safe; otherwise pay tribute. If you refuse to do either, you have only yourself to blame. A people is already upon you loving death as you love life.
Now I don't know if that was big talk or mistranslation, but it doesn't strike me as a good opening gambit. Khalid was not really a monster: being on a Jihad meant that his soldiers didn't get to do the usual victory dance of rape and pillage. They weren't even allowed to kill women, children, religious leaders or men who had not fought.
Most of the Iraqi tribes at the time were Christian, and they decided to pay the tribute and live under Islamic law. Caliph Umar took over and founded two garrisoned cities: Kufah, the new capital of Iraq, and Basra, a port. Umar continued the administrative practices established by the Sassanids (look out for divan or diwan in its Arabic form). Arabic replaced Persian as the official language, and Arabs intermarried with the locals, who gradually converted to Islam. Everything looked set for a rosy future.
The Sunni-Shia Split
The Arabs may have been a unified nation under Islam, but they still lived by their paternalistic code. If the Abu Bakr-Ayesha-Muhammad triangle wasn't enough, wait 'til you see what I got in store for you now.
Muhammad had only been dead a few years, and was time to elect just the third caliph. Uthman encountered opposition both during and after his election to the post. His rival had been Ali ibn Abu Talib, Muhammad's cousin and also his son-in-law, after he married Fatima (no, not "Fatti-ma", "Faah-ti-maa"), Muhammad's only surviving child. (This is interesting, since I'd earlier read that Ibrahim was his only child and Ibrahim died very young. Maybe girlchilds don't count? Maybe someone can clear this mess up?)
Anyway, Ali's pedigree is about as blue-blooded as they come and it turns out he was a snotty little pious chap too. Many groups (who had vested interests of course) preferred Ali to his rival, because they thought it more likely that he would carry on the practises of the previous caliph, Umar. Ali formally opposed Uthman's claim to the office, but drew the trump card by basing his opposition on religious grounds: Ali claimed that Uthman had introduced things that were contrary to Qur'anic laws.
There was a small matter of Economics at play as well. Many Bedouins had offered themselves up for military service in Iraq and Egypt. After the warring, they had gone back to their lifestyles, while the Arabs in the Hijaz (western part of the Arabian peninsula, where you'll find Mecca and Medina) began to live off the fat of the conquered land. When the Bedouins questioned the equitability of the situation, they found a sympathetic ear in Ali.
Groups set off from Iraq and Egypt for Medina to seek redress. Uthman promised reforms, but on their way back to Egypt, the group intercepted a message to the governor of Egypt directing him to punish them upon their return. Uthman was besieged in his home in Medina, and was eventually slain by a son of none other than Abu Bakr. Ali had kept his nose clean in the fray and while the Muslim world was shaken, Ali became caliph.
Here's where it gets interesting: two opponents of Ali set off to demand retribution for Uthman's death, and enlisted the aid of Ayesha, Muhammad's widow, Abu Bakr's daughter and Uthman's slayer's sister. The trio set off for Iraq, but encountered Ali's forces near Basra and the two men were killed. Ayesha returned to her normal life, though trust me there is nothing normal about this incredible woman. She is widely regaled as the wisest woman on earth and was held in particularly high esteem by Muslims in 7th Century Arabia. Enough nodevertising for one day, back to the story.
Mauwiyah, a relative of Uthamn and governor of Syria, refused to recognise Ali. He demanded the right to avenge the death of his kin, which rather than a gentleman's sword fight in later middle ages tradition, was a humdinger of a battle down at the Plain of Siffin, near the largest bend in the Euphrates river. Mauwiyah's forces were on the back foot, so he proposed arbitration. Two arbitrators were chosen to decide whether Uthman's slayer should be deemed an executioner or a murderer, and the arbitrators found the former. Ali then declared that arbitration is contrary to the Sharia, and returned to battle.
The joke was on Ali, however, because a group of his followers, the Kharajites, threw the Qur'an back at him, citing that he was morally wrong to submit to arbitration, and they deserted. Being a reasonable man, Ali appealed to them to return before he slaughtered many of them for refusing. Sensibly, the rest of Ali's army then deserted too. Somehow Ali escaped with his life and hightailed it back to Al Kufah (near Baghdad).
Not surprisingly, when Islamic leaders met to discuss who to elect to the leadership role, neither Muawiyah's nor Ali's name came forward. Umar's son, Abd Allah (more familiarly written as Abdullah) was proposed, but the meeting adjourned without a clear decision. Meanwhile, with Ali and Muawiyah cooling off in Iraq and Egypt respectively, both had been elected caliph by their supporters.
Islam might have recovered from the turmoil quickly, but Ali was murdered in 661 by a Kharajite while praying in a mosque at Al Kufah. Muawiyah tried to capitalise on the situation by persuading Ali's eldest son Hasan to renounce his claim to the caliphate (a task made easier by the fact that Hasan's days were numbered thanks to a spot of consumption). Unfortunately though the "martyr" tag had already been placed on Ali and inspired the Shias (or Shiite or Shiat Ali) to stick with Ali's cause. Muawiyah's mob - the Ahl as Sunna or Sunnis, were equally sticking to their man. The rivalry endures to today.
The Abbasid Caliphate
The family structure gets a little convoluted here so hold your breath, close your eyes and go with trust. Muhammad's grandfather was called Hashim, hence his male-line descendants were known as the Hashimites. Hashim was a descendent of both the Shia line and the Abbas line. Therefore anyone claiming to be of the House of Abbas (so having a name ending in al Abbas) could also claim kinship with the Prophet. The Shia also. The Shias therefore actively supported the Hashemite leaders. Abd al Abbas, not a Shia but a Hashemite, organised a group of rebels under the battle cry of "House of Hashim" and under Hashemite leader Abu Muslim, took on the Umayyads who were resident in Baghdad. In 750 AD therefore, Abd al Abbas became the first caliph of the Abbasid Dynasty. You still with me? Good.
The Abbasid Dynasty lasted until 1258 AD, elevating Baghdad to prominence (it was second in size only to Constantinople, which you may be more familiar with under its earlier name of Byzantium or later name of Istanbul). The growth was thanks to the construction dykes, canals and reservoirs, which contained the mighty rivers and eradicated malaria from the City.
Baghdad became a thriving hub of Arab and Iranian cultures, exporting vast quantities of food as well as philosophical and scientific thought and literature. Of course the Sunnis and Shias were still squabbling, and every time a caliph passed on there was the mandatory infighting, but the Abbasid Dynasty managed to endure even while in the west empires were breaking up. The Iranians got the hump in the early 9th Century, and the Turkish military began to be a bit of a niggle up north.
The Abbasids responded to the Turkish threat by importing Turks as slave-warriors (Mamluks). The Turks began to move upwards through social strata pretty quick, and 833 AD's new caliph, | 
