Between the US and Iran, who started this latest fight?
Like most international conflicts, there is not one defined starting point for the escalation in hostilities.
However, in terms of the most recent increases, this probably links most directly to the US cancelling the six-nation deal which was signed with Iran over suspending uranium enrichment in return for sanctions relief.
Following that, the US cranked up the sanctions against Iran to an extraordinary degree causing catastrophic damage to the Iranian economy (as was intended).
The Iranians responded with a series of aggressive moves within the region by their intelligence services, most notably blowing up several oil tankers in the gulf.
However, this didn’t get much of a reaction from the Americans, so they upped the ante and started attacking American targets more directly. This included the murder of a US civilian contractor, and the attack on the US embassy in Baghdad.
The US did respond to that, with the drone strike on Suleimani and his entourage.
Which takes us up to where we are today.
If the US doesn’t cancel the deal and crank up the sanctions, does all this still happen? Probably not. But equally, it would be niave to assume that nothing would have happened. Iran absolutely loves meddling in the Middle East, and that is pretty much all that its intelligence services do these days. So there is a sensible argument that can be made that eventually tensions would have cranked up to their current level in any event.