Russia has been supporting Syria for years against, well, all comers. Some months ago NATO member Turkey decided to launch a military offensive inside Syria which has heated up of late and became far more serious overnight when Turkey shot down two Syrian fighter jets.

Turkey Shoots Down Two Syrian Fighter Jets as Clashes Intensify: https://www.bloomberg.com/news/artic...shes-intensify

Meanwhile Turkey has been paid to shelter millions of Syrian and other refugees for years. Turkey's leader has asked allies for support in their (illegal) invasion of Syria lest he would open border crossings so that those refugees could attempt to cross into Greece, the gateway to other EU countries. Yesterday he did just that.

Thousands gather at Turkish border to cross into Greece: EU border agency on ‘high alert’ as Turkish president Erdoğan keeps crossings open - https://www.theguardian.com/world/20...ss-into-greece