Vladimir Putin’s Excellent Syrian Adventure was all fun and games so long as he was hitting Bashar al-Assad’s enemies (which rarely meant Daesh until that group downed a Russian airliner) and those enemies couldn’t fire back.
However, Russian pilots had the unfortunate habit of straying into Turkish airspace. This was understandable, as the air space in the war zone was rather tight. But the Turks warned Russia after the last incident that another violation of their airspace would bring about a response.
Today, a Russian jet again strayed into Turkish airspace. According to Turkey and the coalition command in Baghdad, Turkey warned the Russian plane ten times to leave its airspace. The Russians either didn’t hear the warnings, or ignored them, at which point Turkey downed the Russian jet. The pilots ejected, but didn’t survive. (The details of their deaths are still sketchy, but it seems that Syrian rebels shot them down.)
White House.gov