What are the main differences and similarities between the Syrian and Ukrainian crises?
The main similarity between Ukraine and Syria is that they are both relatively recently created nations, with boundaries drawn by fiat rather than evolving naturally, and thus containing many different and often radically incompatible groups. The crises both are currently undergoing are in part due to their fissiparous nature and in part due to outside interference.
The current crisis in Syria is in a large part due to an endogenous civil war, caused by other groups rebelling against Assad, a dictator from the minority Alawite Muslim group. The current state of civil war is due in part not just to a desire to overthrow a brutal dictatorship but to the rebel factions having different goals and often fighting against each other as well as the regime, meaning that there is no easy replacement for the current regime. Complicating the situation is that outside powers are supporting multiple factions, something that may prolong the crisis.
In Ukraine, there is a similar issue of artificial national boundaries, with many areas in the eastern Ukraine speaking primarily Russian and western Ukraine speaking Ukrainian. The current crisis has been prompted by the Ukrainian government, centered in Kiev, moving to closer ties with the EU and Russia responding by invading eastern Ukraine and annexing Crimea. The situation differs from that of Syria in that there are only two major factions involved, and that the aggressiveness of Russia rather than an internal rebellion is the main obstacle to resolution.