Bashar Assad is the dictator of Syria and an Alawite Muslim. Putin maintains an active troop presence in Syria and is potentially ready to fight with America over his ability to sustain Assad in power.
Sometimes, the argument is made that Assad is an ally in the fight against Sunni terrorism. This is a good talking point for some people. However, it runs contrary to the facts.
Assad helped Sunni jihadists back in 2003 to fight Americans in Iraq, he gave his full support to safe housing the future ISIS-founder Abu Musab Zarqawi, and today he is ISIS's largest source of funding by buying their oil, as reported by the Wall Street Journal.
Assad is closely allied with the Shia terrorists, Hezbollah. Bashar Assad is also son of Hafez Assad, together they occupied Lebanon for decades, overseeing the rise in power of Palestinians and Hezbollah, at the expense of Christians. Under the Assad occupation, Christians suffered politically, and dropped from 63% of Lebanon's population to a mere 34%.
Putin cares so deeply about supporting Assad that this extends to the use of chemical weapons killing civilian children.