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By Fareed Zakaria

Here's a startling statistic: more than 8,000 Iraqis were killed in violent attacks in 2013. That makes it the second most violent country in the world, after its neighbor Syria.

As violence has spread and militants have gained ground in several Middle Eastern countries, people have been wondering how much this has to do with the Obama administration and its lack of an active intervention in the region. The Wall Street Journal and a Commentary magazine opinion piece have both argued this past week that the Obama administration's decision to withdraw troops from Iraq is directly responsible for the renewed violence in that country. They and others have also argued that because it has stayed out of Syria, things there have spiraled downward.

Let me suggest that the single greatest burden for the violence and tensions across the Arab world lies with a president – though not President Obama – and it lies with an American foreign policy that was not too passive but rather too active and interventionist in the Middle East. The invasion and occupation of Iraq triggered what has become a regional religious war in the Middle East. Let me explain how, specifically.

From March through June of 2003, in the first months of the occupation of Iraq, the Bush administration made a series of catastrophic decisions. It authorized the disbanding of the Iraqi army and signed onto a policy of deBaathification, which meant that anyone in Iraq who had been a member of the top four levels of the Baath Party – the ruling party under Saddam Hussein – would be barred from holding any government job.

This meant that tens of thousands of bureaucrats and hundreds of thousands of soldiers – almost all Sunnis – were thrown out of work, angry, disposed, and armed. This in turn meant the collapse of the Iraqi state and of political order. But it also sparked the rise of a sectarian struggle that persists to this day.

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The Bush administration went to war in Iraq to spread democracy. But in fact it spread sectarianism – displacing the Sunni elite who had long ruled the country and replacing it with hardline Shia religious parties that used their new found power to repress the Sunnis – just as they had been repressed.

Meanwhile, Iraqi Prime Minister Nuri al-Maliki has been utterly unwilling to share power with the Sunnis – who comprise about 20 percent of Iraq – and that has driven them into opposition, extremism, and terrorism. During the surge the prime minister made several promises to change his ways and over the last few years has reneged on every one of them.

This sectarian power-struggle is the origins of the civil war that has been ongoing in Iraq for 11 years. It is the cancer that has spread beyond Iraq into other countries, from Syria to Lebanon.

The Bush administration seemed to have made the massive strategic error almost unthinkingly. There is for example a report that a few months before the invasion, President Bush met with three Iraqi exiles and appeared unaware that Iraq contained within it Sunnis and Shias. An Arab leader confirmed to me that in his meetings with the president, it was clear that Bush did not understand that there was a difference between the two sects. Others in the administration, better informed, were convinced that the Shia would be pluralists and democrats. Those of us who warned of these dangers at the time were dismissed as pessimists.

So if we're trying to understand why we see a Sunni-Shia battle unfolding across the Middle East, keep in mind that the primary cause is not that the Obama administration didn't intervene in Syria. It's because the Bush administration did in Iraq.