The assassination of Qassem Soleimani in Iraq marks a significant escalation in US-Iran tensions. Retaliation will be forthcoming from the Islamic Republic; failing to do so would risk credibility to the point of posing an existential threat to the regime. The only questions are where the counter-attack will come (Iraq, Israel, Syria, Lebanon, Yemen, to name a few possibilities), when it will happen, and how the United States and its allies will ultimately respond.
The dominos are now lined up. Cascading events will at best produce a new wave of instability in the Middle East, one that is fueled by political events that were already in motion well before the Soleimani killing. At worst they will result in a new US war in the Middle East.