Ankara has fought ethnic Kurdish militias within its borders for decades to prevent the birth of a new country, Kurdistan, carved out of Turkey itself. So it has watched carefully as YPG fighters secure large swaths of land along the Syrian-Turkish border.
Kurdish fighters carry flags near Raqqa in Syria after they captured the area on June 15, 2015. Rodi Said / Reuters
To Turkey’s government, these Kurds are no different to the outlawed PKK militants who have carried out a series of deadly bombings within Turkey in the last year. The group is trying to force Turkey to accept the Kurdish right to an independent state, which has been denied them for a century.
Source
http://www.nbcnews.com/storyline/isis-terror/battle-mosul-why-turkey-bombing-anti-isis-fighters-iraq-syria-n670511
