Close after the very birth of Islam as a religion, the division entered the belief system. The prophet Muhammed, Islam's primary holy figure, died without naming a successor to lead the religion and subsequent empire. The Sunni branch of Muslims believed Islamic leaders should elect an heir, while the Shia branch believed the heir could only come from Muhammed's family. Ever since then, intense and often violent conflicts have taken place between the two sects.  The ruling Assad regime of Syria is Shia, while a majority of citizens are Sunni. This division has been a large driving force in the Syrian civil war. 
The Middle East's drastic sectarianism has lead to an immense crisis in Syria, and the only way to solve the crisis is for the root division to be dealt with. The Islamic State's brutal occupation and violence are a result of the religious tensions between the Sunnis and Shiites, and Syria's Middle Eastern neighbors must band together despite their differences in order to solve the issue. The Islamic State uses the instability in Syria and exploits it to further their terrorist activities; the sectarianism in the Middle East is what gives IS the opportunity to spread bloodshed. Some states, such as Iraq and Iran, back the Shiite Assad regime, being Shiites themselves. Others, such as Saudi Arabia and Turkey, back the Sunni opposition groups. The use of the Syrian battlefield in this way increases violence in the region and exacerbates the crisis. The only way to stop the IS, and subsequently the Syrian crisis, is for regional countries to stop backing groups in Syria to push their religious sect's prominence, and instead work together to solve the crisis.
Solving a 1400-year-old conflict between religious sects is an unrealistic way to solve a time-sensitive crisis. Resolving the sectarianism in the Middle East would be great, but it is a decades, possibly centuries-long task. Millions of Syrians are dying and being misplaced right now, and resolving the deep rotted issue at play would take too long. An immediate solution is more realistic and helpful to solve the crisis.
[P1] The root of the Syrian crisis is sectarianism. [P2] Sectarianism in Syria is expanded by regional countries. [P3] Regional actors discontinuing the sponsorship of sectarianism in Syria will end the crisis.
Rejecting the premises
[Rejecting P3] Discouraging sectarianism is not a realistic goal to end the crisis.