Moscow's doubts about Turkey will be rekindled
In less than a year, Russia and Turkey have gone from the brink of war to one of the most potentially important alliances in global politics.
Now the assassination of Russia's ambassador will test the growing trust between Vladimir Putin and Recep Tayyip Erdogan to the limit.
To call the relationship between Russia and Turkey "fraught" would be an understatement.
The two have fought dozens of wars over centuries. Yet they also share thriving economic ties.
The Syrian civil war, where they back opposing sides, has thrown these contradictory urges into sharp relief.
When Mr Erdogan ordered his air force to shoot down a Russian bomber in November 2015, it sparked a crisis that nearly spilled into war.
Mr Putin slapped sanctions that dealt a body blow to Turkey. Then, in June this year, Mr Erdogan said the hardest word, telling Mr Putin he was "sorry" for the shoot-down. As if by magic, the antagonism vanished.
Mr Putin then turned a blind eye to Mr Erdogan's land incursion into Syria and backed the Turkish president after a failed military coup.
In return, Mr Erdogan reined in Turkey-affiliated rebel groups. But last night's murder by a policeman is a reminder of just how strained that alliance will be.
The policeman who shot Mr Karlov shouted slogans associated with al-Nusra (now Jabhat Fateh al-Sham), the al-Qa'ida-affiliated group seen as one of the most powerful anti-Assad factions.
That may rekindle doubts in Moscow about where the loyalties of many Turkish officials lie. (© Daily Telegraph London)