Walking among the starvation cells and gas chambers where more than a million souls perished, it is hard to explain to my 12-year-old son how the free world allowed this happen. Why did the world’s democracies not stop the Nazi movement in its infancy — before it indoctrinated millions with its hateful ideology, took control of a great power, built its military might, invaded foreign lands and constructed death camps like the one here at Auschwitz?

The depressing answer is: for the same reason that the world’s democracies have done almost nothing to stop the rise of the Islamic State.

It is difficult to describe the feeling of spending a day at Auschwitz and then watching the images emanating from the Middle East, where the Islamic State is carrying out acts of unspeakable barbarism — beheading unbelievers; sticking decapitated heads on posts; burying women and children alive; randomly shooting pedestrians and motorists; even crucifying — yes, crucifying — its opponents.

So proud are the Islamic State’s leaders of these crimes against humanity that they broadcast them for the world to see on Twitter and YouTube. In one propaganda video, they show hundreds of victims being trucked to an execution site, where they are filmed pleading for their lives before being marched toward open pits and shot one by one. Islamic State militants recently tweeted a photo of a decapitated head with this message: “This is our ball. It is made of skin #WorldCup.”

When it takes over Christian neighborhoods, the Islamic State paints the letter “N” on the homes and businesses of Christians (marking them as followers of the Nazarene, a pejorative reference to Christians in Arabic) before confiscating them and giving them to Muslims. It runs camps where it indoctrinates children to believe that all non-Muslims are sub-human, “apostates” and “infidels” who should be exterminated. Vice News recently interviewed children undergoing such indoctrination by the Islamic State. One young boy looks into the camera and says “In the name of God my name is Daoud and I am 14 years old. I’d like to join the Islamic State and to kill with them, because they fight infidels and apostates.” Another declares: “We promise you car bombs and explosives. . . . I swear to God, we will divide America in two.”

Some would like to believe that the Islamic State’s offensive in Syria and Iraq is nothing more than “a quarrel in a faraway country between people of whom we know nothing,” in the words of Neville Chamberlain. In fact, the Islamic State’s leaders have a messianic vision of building a totalitarian Islamic empire encompassing all current and former Muslim lands, stretching from Europe to North Africa, the Middle East and Southeast Asia. They may never see that vision realized, but they can wreak an awful lot of death and destruction trying.

Their “caliph,” Abu Bakr al-Baghdadi, recently warned: “Our last message is to the Americans: Soon we will be in direct confrontation, and the sons of Islam have prepared for such a day. So watch, for we are with you, watching.” They are rapidly accumulating the recruits and resources to follow through on this threat. The Islamic State is actively establishing cells outside Iraq and Syria, including in Europe. And according to scholars at the Rand Corporation, it “currently brings in more than $1 million a day in revenue and is now the richest terrorist group on the planet. . . . A conservative calculation suggests that [the Islamic State] may generate a surplus of $100 million to $200 million this year.”

To put this in perspective, al-Qaeda spent about $500,000 to carry out the Sept. 11, 2001, terrorist attacks.

The worst part is that all of this could have been prevented. Just a few years ago, the Islamic State (then al-Qaeda in Iraq) was a spent force, defeated both militarily and ideologically thanks to the 2007 U.S. surge, and the Sunni masses who rose up to join the United States in driving them out. Then President Obama’s complete withdrawal of U.S. forces in 2011 took the boot off of the terrorists’ necks. And, as Hillary Clinton recently pointed out, Obama’s “failure” to act in Syria “left a big vacuum, which the jihadists have now filled.”

Now, after standing by and allowing the Islamic State to established control in an area the size of Belgium, Obama has finally launched limited strikes — but only to prevent the Islamic State from overrunning U.S. diplomatic facilities in northern Iraq (for fear of another Benghazi), massacring Yazidis and controlling the Mosul Dam. Obama insists that “there’s no American military solution” to the rise of the Islamic State and that “it’s time to turn the page on more than a decade in which so much of our foreign policy was focused on the wars in Afghanistan and Iraq.”

Obama may be tired of war, as were Hitler’s enemies. But the Islamic State is not tired of war. It has been explicit about its intentions. The lessons of history are clear.

The free world ignores such barbarity at its peril.

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