There is a certain irony in the conquest of much of Syria by ISIS. After 9/11, the Assad regime declined to join the American “global war on terror” (GWOT) in any serious way. Instead, it harbored Sunni Islamists. After the American invasion of Iraq in 2003, large numbers of foreign fighters passed through Syria on their way to join Abu Musab al Zarqawi. One Islamist leader explained Assad’s tolerance for these terrorists: “we [are] focusing on the common enemy, America and Israel.”
In 2007, the balance of forces in Iraq suddenly shifted. Zarqawi’s fundamentalism and his savagery had estranged many Sunnis in Iraq. This led to the “Awakening” movement that greatly reduced the need for American forces while offering much intelligence to the American Special Forces man-hunters. The George W. Bush Administration surged in reinforcements that allowed the US to restore order in Iraq and to pursue the Islamists. The situation began to improve. The Americans killed Zarqawi. Soon, his surviving followers took shelter in eastern Syria, beyond the reach of the man-hunters and the bombs. This allowed many American decision-makers to start looking for an eventual escape route. For his part, Assad seems to have started rounding-up Syrian Islamists whose usefulness had now declined.
Then came the “Arab Spring.” Popular uprisings—generally non-violent—began against the tyrants who ruled (and still rule) much of the Middle East. These movements rocked Tunisia, then Egypt, then Syria, and then Libya. The Tunisian regime soon struck its tents, but it took various types of American pressure to bring “reform” to Egypt and Libya. America had no such leverage in Syria.
At first, Bashar al-Assad responded to the popular challenge by force. This might well have done the job if he had stuck to his last. His faced a loose coalition of talkers-more-than-doers who were often at odds with one another. Like the young Egyptians of Tahrir Square, they seem to have had little support among the populace at large.
Instead, however, Assad tried to tar the rebels as Islamists. To this end, he released a lot of experienced Islamists from his jails. As expected, they took up arms against the regime. Assad then cast his government as the only viable barrier against jihad. Meanwhile, the surviving Iraqi Islamists had reconstituted themselves in eastern Syria as ISIS, with Abu Bakr al-Baghdadi as their leader. As the civil war dragged on, ISIS took control of much of the eastern part of Syria. Then, in Summer 2014 it attacked into western Iraq, routing Iraq’s army.
The results of Assad’s policies has been appalling. Huge numbers of deaths, hordes of miserable refugees, and a society laid in ruins. Many observers regret that the powers had intervened early on to replace Assad and create some kind of viable successor state. There are reasons to question this view. On the one hand, Assad followed a particularly disastrous version of the same course that is being followed more successfully by Egypt. There the army turfed the Muslim Brotherhood out of power and has used the struggle against radical Islam as cover for a revived military dictatorship. So far, that approach seems to be working, mowing down young secular opponents of the old regime with as much enthusiasm as Islamists. So, it was not a foregone conclusion that Assad’s policy would fail.
On the other hand, the “coulda-woulda-shoulda” view ignores the reality that the Syrian civil war is a proxy war for Shi’ites and Sunnis. It also ignores the reality that Russian agreement to yet another American intervention-overthrow would have been necessary to get UN approval. That wasn’t likely to happen after the Libyan imbroglio.
 Charles R. Lister, The Syrian Jihad: Al-Qaeda, the Islamic State, and the Evolution of an Insurgency (OUP, 2015)