I think the most likely outcome of this is the collapse of the whole U.S. electrical Grid from Iran

If you have watched the 2nd season of FUBAR on Netflix this is the most likely result of the whole power grid (or large sections of it) beingbrought down by the Iranians here in the U.S. and possibly Europe too and Israel too. This would NOT be a military attack but a type of virus attack. There would be no boots on the ground that were recognizable Iranian Troops.

But it would be done differently than in the 2nd Season of FUBAR with Arnold on Netflix. IT wouldn't be done physically but rather over the World Wide Internet and this could happen to the U.S. and to Europe as a result of all of this and could take years to repair. This is a good time to have solar cells on your roof and some kind of battery to store your electricity for example. Without solar cells  here in the U.S you might be without power for a long long time (months or years) in your area of the country or world.

Much of our Grid system is very antiquated computer wise and is very very vulnerable to attack from Iran or other people like in Yemen and any other Shia nations now.

Another result is that Saudi Arabia could attack Iran because they have been Angry at Iran for hundreds of years and are in sort of a Religious war with them for hundreds of years (for example, the bombers of 9-11 were all from Saudi Arabia).  Also, Iran has been paying the Houthis to continue to attack and send missiles into Saudi Arabia for years now.

However, now that this has happened The Saudis might wipe out the Houthis and the Iranians now Iran is weakened like this. So, the whole middle east likely will become a bloodbath now of Sunni against Shia Muslims which means millions of people could now easily die now that Iran has been weakened like this. 

So, Trump just stepped into a religious war that has gone on for centuries between the Saudis and the Shias of Iran, Yemen, and Syria and Iraq.

One outcome could easily be the complete genocide of anyone Shia in the Middle East by Saudi Arabia and other Sunni nations. 

To Trump I would say: "People who live in Glass houses shouldn't throw stones" (the stones being 30,000 pound bunker buster bombs).