The left has never adapted to the transition from nationalistic wars to ideological wars. It took the left a while to grasp that the Nazis were a fundamentally different foe than the Kaiser and that pretending that World War 2 was another war for the benefit of colonialists and arms dealers was the behavior of deluded lunatics. And yet much of the left insisted on approaching the war in just that fashion, and had Hitler not attacked Stalin, it might have remained stuck there.
The Cold War was even worse. The left never came to terms with Communism. From the Moscow Trials to the fall of the Berlin Wall, the moderate left slowly disavowed the USSR but refused to see it as anything more than a clumsy dictatorship. The only way that the left could reject the USSR was by overlooking its ideology and treating it as another backward Russian tyranny being needlessly provoked and pushed around by Western Europe and the United States.
Having failed the test twice, it is no wonder that the left has been unable to come to terms with Islam, or that it has resorted to insisting that, like Germany and Russia, the Muslim world is just another victim of imperialism and western warmongering in need of support and encouragement from the progressive camp.
The anti-war worldview is generations out of date. It is mired in an outdated analysis of imperial conflicts that ceased being relevant with the downfall of the nation-state and its replacement by international organizations and causes based around ideologies. Nazism could still loosely fit into the jackboots of the nation state. Communism was another creature entirely, a red virus floating around the world, embedding its ideas into organizations and using those organizations to take over nations.
Islam is even more untethered than Communism, loosely originating from powerful oil nations, but able to spring up anywhere in the world. Its proponents have even less use for the nation state than the Communists. What they want is a Caliphate ruled under Islamic law; a single unit of human organization extending across nations, regions and eventually the world.
The left is incapable of engaging with Islamism as an ideology, instead it reduces the conflict to a struggle between colonial and anti-colonial forces, showing once again that the left's worldview is usually at least fifty years out of date.

Their response to the Clash of Civilizations has been to include Islamists in the global rainbow coalition of minorities, gays and gender theorists, indigent third world farmers, transsexuals, artists and poets, sex workers and terrorists; without considering what the Islamists were or how they would fit into this charmed circle.
The left views the Islamists as just another front group to be used. 
The Islamists see the left the same way and in Iran, Egypt and Tunisia, the Islamists have a better track record of getting the better of the left. But the left never learns from history. It never questions its outdated Marxist fisheye view of events or realizes that the Industrial Revolution, feudal peasants and the banks are not a metaphor for absolutely every struggle that takes place anywhere in the world. 
And so the left dooms itself to repeat again and again the history that it refuses to learn.
Rather than deal with Islamism, the left persists in fighting phantom wars against nationalism, capitalism, militarism, colonialism and imperialism; all things that are approaching extinction in its sphere of influence, while thriving outside its sphere of influence. 
The left is too busy fighting a civil war to see that if it wants to survive, it will have to fight a global war. True to its nature, it is determined to finish digesting the West before it is ready to defend it, and by the time that the left digests the West, with the help of its Islamist allies, the war will be over and the left will have lost.
The left is undone by its own conception of history as a treadmill moving forward through historical stages, rather than a chaotic morass of forces colliding together. In the progressive understanding of history, progressive forces defeat reactionary forces and humanity advances to the next stage. There is no room in that neat orderly evolution for the violent chaos of Islamism and its resurrection of tribal forces, ethnic grievances and religious intolerance into a worldwide movement that is every bit as fanatical and determined to forcibly carve out its own vision of a new world order.
From the progressive perspective of history as an evolutionary process, Islamist tribal fanaticism is from too early a stage to threaten the left. Socialism must battle against the industrialism of the previous stage, with each generation advancing the future by destroying the achievements of the previous generation in a species of grim historical cannibalism. The left fears being held back by capitalism, not by Islamism. It does not believe that the values of the 6th century can compete with it, only that the values of the 19th century can.
While the left sees itself as progressive and Islamism as reactionary, it is the left that has trouble adapting to new developments, while the Islamists have successfully glommed onto everything from the Cold War to the fall of the Soviet Union, the rise of international organizations and even the War on Terror, and exploited events for their ends. In the new century, the Islamists have been riding the left over the finish line, without the left realizing that it was being ridden.
The left dwells in an intellectual bubble of its own making. 
It transforms that bubble into an elaborate place, furnishing the space until it resembles a miniature world, but a bubble is not a world, it can only ever be a bubble. Trapped inside the bubble, the left cannot realize that the world is going backward, not forward, that the 21st century is really the 7th century and that the future is the past.
The Islamists understand this quite well. The left cannot.