Race, ethnicity and religion are always important factors in the politics of a given country. Modern Syria was formed out of a province of the Ottoman empire with Lebanon carved out in turn by the French as its own country.
Arab Nationalism was a force in the Arab world in the last century which tried to transcend religion and was focused on anti-colonialism. The Baath parties, for instance, focused on Arab nationalism, but religion was still there. In Syria we had a Baath party centred on a ruling family drawn from the small Alawite minority while in neighbouring Iraq there was a Baath party led by that country's Sunni minority. Assad's Syria had complicated regional relations. They supported Iran in the Iran-Iraq War, but they supported a Christian government in Lebanon at the start of the Lebanese Civil War against the PLO. Opposition to Israel, though, was constant and the occupation of the Golan Heights prevented peace accords that had been struck between Israel and Egypt and Jordan.
Assad's regime kept a tight grip on domestic politics (including the massacre of something like 40,000 Sunnis at Hama in 1982) up until the Arab Spring of 2011. Once the Sunnis were mobilized the Assad regime had to rely on Russian and Iranian support (including Hezbollah fighters from Lebanon).
Syrian demographics are estimated as follows:
Ethnicity: Arabs are 85% of the population, Kurds 10% of the population with various other groups filling out the remainder. Some of those were displaced into Syria during the Ottoman times.
Religion: Sunni Muslims are estimated to be 75% of the population, with Shia/Alawites comprising something like 12% of the population, Christians close to 10% and the Druze around 3%.
All this to say, the demographic cards were heavily stacked against the Assad regime once the Sunni majority was mobilized. The Kurds are Sunni, but are seeking to have control over their own destiny. How this plays out in the coming months may depend on how much the US continues to support the Kurdish forces in the east of Syria (Syrian Democratic Forces - SDF). Turkey is certainly opposed to the SDF, so while the Assad regime has fallen the civil war is likely to continue.
All that to say, the situation is different than faced by the Sykes-Picot team when they carved up the Ottoman Empire. Unlike 1916-18, the inhabitants have agency and are fighting for their future.