1. The Obama admimistration has been funding ISIS indirectly to destabilize syria. Trump may... or may not be. His administration has been... or perhaps has given the appearence of being schizophrenic on this count.
2. It gives Iran excuse to expand into Syria and Iraq, just as 9/11 allowed us to hit Afghanistan. Unlike Afghanistan, this is just over the border for Iran.
This leads to 3.
3. America, in my opinion, is unlikely to allow Iran to expand and gain power within the region, as their foreign policy has had a goal of consistently opposing any sort of arab hegemonic power*.
So there you have it... a tinderbox waiting for a spark. Much depends on irans next move. They also it should be noted have one of the largest standing militaries in the world.
*(Apart from our bestest friends isreal.)
1. I would like to see some (non TFH) proof of that accusation please.
2. Iran may have relatively easy access to Iraq however they would have to go through most of the Iraqi army, which happens to be in the area, to get to where ISIS is, let alone Syria. This would require a major invasion on the Iran's part, which also brings me to
3. The US would oppose, effectively, an invasion of Iraq by Iran, and likely with heavy international support. China would stay out and Russia would be alone in their half hearted monetary support, the rest of the Middle East would fall into line with the US, just like in the Iran-Iraq war as the last thing ANYONE in that part of the world wants is Iranian hegemony.