Final answer:
The Assad regime (Alawite sect) and Sunni Muslim extremist groups, including ISIS, represent the two sects most at odds in Syria's conflict.
Step-by-step explanation:
In Syria's ongoing conflict, the two sects that appear to be most at odds with each other are the Assad regime, represented by the Alawite sect, and various Sunni Muslim extremist groups, including ISIS. The Assad regime has been in power for over 40 years with an authoritarian grip, maintaining control with a significant military presence and support from allies such as Russia and Iran. In contrast, Sunni Muslim extremists aim to establish a theocracy based on their interpretation of Sunni Islam, which opposes Assad's secular rule. The complexities of the conflict are compounded by various other factions and international involvements, but these two groups represent the central opposing forces within Syria's war.