Turkey and Russia, whose up-and-down relationship has helped shape the Syrian war and its related crises, shared a new trauma on Monday after an off-duty Turkish police officer assassinated Russia’s ambassador.

The killing of Andrey G. Karlov in Ankara caused global alarm and raised questions about the larger consequences.

On social media, many drew parallels to the assassination of Archduke Franz Ferdinand of Austria, which helped set off World War I — a comparison that analysts, thankfully, reject.

No group has claimed responsibility for the attack, which took place at an art gallery in the Turkish capital where Mr. Karlov was speaking. The gunman, who was killed at the scene by security forces, shouted “God is great” in Arabic and later said in Turkish: “Don’t forget Aleppo. Don’t forget Syria.”