The American Interest points out:
By all accounts, the Iraqi Army, or ISF, collapsed in the defense of Ramadi, just as it has time and again against ISIS previously, abandoning arms and armor to the enemy as it fled. The Shi’a militias are a more feared fighting force, and they outnumber the ISF by a significant margin. They had been held back, however, because Ramadi, the capital of Anbar province, lies in the heart of Sunni Iraq—and the Shi’a militias have been repeatedly, credibly accused of perpetrating sectarian massacres. And there is also the inconvenient fact that many if not most of them have strong links to Iran.This is just another result of the First World War. When the allies dismembered the Ottoman Empire, they pretty much demarcated the boundaries of the new nations out of political expediency or to award petty tribal rulers for their assistance, rather than based on ethnicity or tribal boundaries. A nice set of maps illustrating the stages--from the Ottoman Empire, through the 1916 Sykes-Picot Agreement, and the final 1920 San Remo agreement--can be found here. The article, "The Disintegration of the Iraqi State Has Its Roots in World War I" at Smithsonian Magazine provides further details. It recounts:
Now the Obama Administration, not to say the Iraqi government, is on the horns of an ugly dilemma. If Ramadi is not recaptured, Sunni Iraq will have slipped to ISIS, and all the king’s horses and all the king’s men may never be able to put Iraq back together again. On the other hand, if the U.S. backs the militias’ advance, it may well be party to ethnic bloodshed that will put the killings after the fall of Tikrit to pale. Thus, even if the militas do retake Ramadi the methods they employ could so deeply antagonize the non-ISIS-supporting elements of the Sunni population as to have the same result: no more Iraq.
In order to raise an Arab revolt against the Ottomans, who had joined with Germany and Austria-Hungary in World War I, Great Britain forged a wartime alliance with Emir Hussein of the Hejaz region of Arabia, now the western edge of Saudi Arabia bordered by the Red Sea. The 1915 pact was a mutually advantageous one. Since Hussein was an extremely prominent Islamic religious figure, the guardian of the holy cities of Mecca and Medina, the alliance inoculated the British against the Ottoman accusation that they were coming into the Middle East as Christian Crusaders. In return, Britain’s promises to Hussein were extravagant: independence for virtually the entire Arab world.Winston Churchill was appointed to oversee the problem, which was partially resolved by creating Lebanon and Saudi Arabia as kingdoms and placing the power in the hands of powerful warlords who had assisted the British. However, Iraq remained restive until Britain was forceably expelled in the 1950s. (Although Iraq had its own king, he had been chosen by the British and regarded as a British puppet). The political stability under the Baathist's iron fist was destroyed by the U.S. invasion, but to the U.S., dividing the country had never been an option.
What Hussein didn’t know was that, just months after reaching this accord, the British government secretly made a separate – and very much conflicting – pact with their chief ally in World War I, France. Under the terms of the Sykes-Picot Agreement, the future independent Arab nation was to be relegated to the wastelands of the Arabian peninsula, while all the most politically and commercially valuable portions of the Arab world – greater Syria, Mesopotamia – would be carved into British and French imperial spheres.
This double-cross was finally laid bare at the postwar Paris Peace Conference in 1919, and solidified at the San Remo Conference in April 1920. Under the terms of these imperial agreements, France was to be given much of greater Syria – essentially the modern-day borders of that country, along with Lebanon - while the British would possession of the vast swath of the Arab world just below, an expanse stretching from Palestine in the west all the way to Iraq.
But if history has shown that it’s always risky to divide a historical homeland, as the British and French had done in greater Syria, even more perilous is to create an artificial nation – and this is precisely what the British had done in Iraq.
In the promises made to Emir Hussein back in 1915 regarding future Arab independence, one of the very few “modifications” the British asked for was in the two southern vilayets of Iraq, where oil had been discovered; here, London suggested, “special administrative arrangements” would have to be made.
By war’s end, however, oil had also been discovered in the vilayet of Mosul, just to the north, and Britain cast its covetous gaze there, as well. Since the promise of Arab independence was already a dead letter, the solution was quite simple: the “nation” of Iraq was created by fusing the three Ottoman provinces into one and put under direct British control.
Naturally, Britain didn’t present this as the land-grab that it truly was. To the contrary, there was much high-minded talk of the altruistic nature of their mission, of how, after a sufficiently civilizing period of Western tutelage, the locals might be allowed to govern themselves. When the ungrateful locals balked at this notion, the British simply dismissed the officials and bureaucrats of the former regime, ignored the tribal leaders, and placed their new vassal state under the direct administration of British civil servants and soldiers.
To the few Britons who actually had some familiarity with that corner of the Arab world, the signs of impending calamity were unmistakable. Among them was T.E. Lawrence, better known as “Lawrence of Arabia.” As Lawrence wrote to a newspaper editor in September 1919 in regard to the simmering tensions in Iraq, “if we do not mend our ways, [I] will expect revolt there about March next.”
Lawrence was only off on his timetable, with the revolt actually coming in June 1920. Caught completely off-guard was the local British administration. Within weeks, hundred of their soldiers and civil servants had been killed, with the rebellion only eventually put down by a “surge” of British troops and severe military reprisals, including the dropping of poison gas on tribal insurgents.
The result over the past decade has been the gradual dismantling of the Iraqi nation. Long gone, either to their graves or to foreign exile, have been the country’s relatively small communities of Christians and Yazidis, adherents of a religious splinter sect in northern Iraq long derided by both Sunni and Shiite Muslims as “devil worshippers.” Most devastating has been the eruption of the Islamic Shia-Sunni schism into sectarian slaughter. Vast swatches of the Shiite-majority regions of southern Iraq have been “ethnically-cleansed” of their Sunni minorities, while precisely the same fate has befallen the Shiite in Sunni-dominant regions. This purging has extended down to the village, and even city neighborhood, level. Amidst this quagmire, the Kurds of northern Iraq, who long ago effectively seceded from the rest, are establishing their own government complete with their own military and border controls. For those who, in 2003, worried that the American mission in Iraq might become an extended exercise in “nation-building” precisely the opposite has proven true.A somewhat lengthier article, focusing on the events immediately after WWI, is "Iraq: Historical Setting-Library of Congress Country Study-World War I and the British Mandate."
In short, the collapse of Iraq was inevitable. How it collapsed was not.
In any event, there is the sobering failure of the Iraqi military to deal with ISIS. As noted in the American Interest article, the Iraqi troops were completely shattered, abandoning weapons as they fled. The Associated Press has more on this topic:
Iraqi troops abandoned dozens of U.S military vehicles, including tanks, armored personnel carriers and artillery pieces when they fled Islamic State fighters in Ramadi on Sunday, the Pentagon said Tuesday.While the Pentagon is, at least publicly, expressing confident that Ramadi will be retaken, I am not. Unless the Iranians intervene militarily, I believe that we will be reading about the battle of Baghdad by summer's end, if not sooner.
A Pentagon spokesman, Col. Steve Warren, estimated that a half dozen tanks were abandoned, a similar number of artillery pieces, a larger number of armored personnel carriers and about 100 wheeled vehicles like Humvees. He said some of the vehicles were in working condition; others were not because they had not been moved for months.
This repeats a pattern in which defeated Iraq security forces have, over the past year, left behind U.S.-supplied military equipment, prompting the U.S. to destroy them in subsequent airstrikes against Islamic State forces.
Asked whether the Iraqis should have destroyed the vehicles before abandoning the city in order to keep them from enhancing IS's army, Warren said, "Certainly preferable if they had been destroyed; in this case they were not."