Since 90's, Ba'ath Party (Ba'ath established by Bashar Assad's father, ''Hafez Assad'') supported PKK terrorists against Turkey. There was PKK camps all over the Northern Syria, and PKK used this camps to attack Turkey. In 90's we declared it as a Casus Belli and Assad's father backed down a little bit, still their support to PKK continued from background. Financial support, intelligence support and of course free weapons.
Anyway, it is not a Erdogan - Assad thing. Ba'ath is a disease, needs to be cured. And Assad scum must understand the consequences of supporting terrorists against Turkish Nation.