According to apologists of Bashar al-Assad, and those they have managed to dupe, he has been making war for the past ten years against terrorists, and any Western attempt to engineer his ouster would be to let the terrorists win. But in fact Assad did much to help Islamic State (IS) come into being, and to keep it alive. Matthew Levitt writes:
One key tactic the Assad regime employed was to focus its military efforts against the moderate Syrian rebel groups opposing the Assad dictatorship, in particular the Free Syrian Army (FSA), and not Islamic State. . . . [Moreover], Syrian intelligence agencies were deeply involved in the Assad regime’s efforts facilitating and providing cover for the terrorist pipeline that ran through Syria into Iraq and helped build up al-Qaeda in Iraq, which later became IS. . . . In fact, this relationships [went] back to 2001-2002.
In May 2011, in the wake of some of the early Arab Spring protests in Syria, the Syrian government began to release hardline Islamist terrorists [from its prisons] in the first of a series of official government amnesties. . . . At the same time, the regime was arresting large numbers of peaceful protestors, students, and human-rights activists. Several of the terrorists released in these first amnesties went on to head Islamist extremist groups in Syria, including Islamic State.
One reason the Assad regime may have elected not to target Islamic State positions in eastern Syria was the regime’s business dealings with the group. The regime purchased oil from Islamic State, and bought wheat from areas under the group’s control, which IS was able to tax, thereby profiting from the transactions with the Assad regime.
While Islamic State remains an insurgent threat in Iraq and Syria, and a global threat as a terrorist network, it no longer controls significant territory and represents a fraction of the threat it once did. But there is no clear global coalition—neither political nor military—to address the threat posed by the Assad regime, which has killed exponentially more people than Islamic State, facilitated that group’s terrorist activities, and caused population displacement, migration flows, and tremendous regional instability.
Read more on Washington Institute for Near East Policy: https://www.washingtoninstitute.org/policy-analysis/role-islamic-state-assad-regimes-strategy-survival