Rebellion and retreat

As centralised order in Iraq falls apart and the British government 'considers' sending Chinook helicopters to assist and evacuate the refugees (what is there to consider?  Just send them.  Valuable time has already been lost through pathetic hand-wringing), the rise of Islamic State (ISIL) is not unlike another long-lasting violent insurrection against a tired and weak status quo - the Taiping Rebellion.
The Manchu Qing dynasty in China was already on its death spiral from the depredations and exactions of earlier corrupt officials that the emperors failed to rein in when this revolt first manifested itself in 1850.  Many irruptions and secret societies marked the final century of Qing imperial rule but this was the most serious of all as the aim was the conquest of all China.  The Qing administration was already viewed with suspicion by Han Chinese as a foreign imposition that did not the interests of the general population at heart and so the Taiping rebels gained support in this way, much like disaffected Sunni Iraqis, alienated by an exclusivist Shia-led government in Baghdad, flocking to the black flag of the Islamic State.
What makes the Taiping rebellion truly comparable is the religious element.  Just as a new caliph has been proclaimed in Mosul when it fell, so the Taiping leader, Hong Xiuquan, declared himself to be the younger brother of Jesus, with a capital at Nanjing.  Hong's fanatical armies exterminated all those who did not conform to this brand of Christianity, including Chinese Christian folk religion, just as Shias, Christians, Yazidis and even Sunni Arabs suffer at the hands of ISIL.  The operation of a theocratic and militarised rule is present both then and now, with a haphazard and brutal application.
Initially, Europeans decided to stay neutral as the Taiping Heavenly Army superior generalship defeated the Qing armies thrown against it.  The Americans and the West in general sitting on their hands as ISIS became ISIL is an all too obvious parallel, just as the decision to intervene to protect Irbil in Kurdistan echoes the involvement of Britain and France, helping protect Shanghai and reorganising the Chinese armies to put down the rebellion, Charles 'Chinese' Gordon leading the way (Gordon would meet his match and his death against another fanatical force in the Sudan).
The strategy of the Taiping Heavenly Army was to take major cities, consolidate their hold on these, then subdue and recruit in the surrounding countryside while engaging Qing forces.  It almost seems that ISIL has been following the same handbook.  The death toll over 15 years of fighting is hard to gauge but most estimates put it between 20-30 million dead, soldiers and civilians, largely from disease and famine created by the dislocations.  A man-made catastrophe, like in Ethiopia in the 1980s and like the Yazidis facing starvation on Mount Sinjar.  So the Taiping Rebellion was suppressed but at great cost.  With ISIL straddling both Iraq and Syria it is hard to see how the endgame will play out, though the fall of Nanjing still left Heavenly Army contingents in the field, the latter making incursions into Siam.
But ISIL could not have made their advances were it not for the failures at the centre.  Nouri al-Maliki has been sidelined as prime minister by his own coalition but much of the Iraqi Army still answers to him.  He is a weak man who wants to be a strongman, regardless that he has been cast aside even by his regional backer Iran.  In the three years since the US Army left, he has hollowed out the Iraqi Army to the same extent that it took several corrupt and/or poorly led governments in Ukraine a quarter of a century to achieve in their own case.  The Ukraine is instructive in other ways as well.  Following the purges of the Soviet Red Army in the 1930s, Marshal Semyon Mikhailovich Budenny came to command the Soviet forces in Ukraine and Bessarabia.  To the soldiers under his command, he was the man 'with the very large moustache and the very small brain'.  As the historian Geoffrey Regan puts it in The Guinness Book of Military Blunders, Budenny's "meteoric rise to a position where he could do such damage stemmed from his friendship with Joseph Stalin and his obvious lack of ability... The fact that Budenny survived [the purges] and prospered speaks volumes."  Senile at 58, despite outnumbering the Germans between three and four to one in men and tanks, his command was annihilated by the invading Germans, losing 1,500,000 men between July and September 1941 alone.  al-Maliki's handpicked choices to lead the Iraqi Army have been utter failures, 350,000 troops fleeing in the face of a force fifty times smaller.  There the comparisons end.  Much of the Soviet equipment destroyed was obsolete and the factory complexes in the Urals would soon restock the Red Army with superior tanks, aeroplanes and automatic weapons.  Unfortunately, much of what the Iraqi Army has abandoned in its headlong rush from the frontline (clearly al-Maliki's cronies had no idea how to instil morale) was top-of-the-range hardware that the USA had sold to Baghdad.  Humvees and tanks are now being driven around by ISIL militants with vast quantities of stores available to them to continue their offensive - a criminal state of affairs.  No wonder the Kurdish Peshmerga militia struggled manfully but ultimately found ISIL irresistible, until the US airstrikes came along.  The USSR prevailed over the Wehrmacht but at a human cost equivalent to the Taiping Rebellion.  ISIL will probably be overcome - eventually - by a combination of government and Kurdish deployments aided by Western air superiority yet with each passing day the casualties mount inexorably.