I think this is a very scary question, but is always a possibility of happening. Iran has nukes, and so does the US, so if the US would go to war with Iran directly, we might see the very last war in history as we know it.
Lets go down the line and see what would happened if USA would go to war with Iran:
Phase #1President Obama goes live on public TV and announces that the security of America is on high alert, and tells us all not to worry or panic, that they will do everything they can to protect our freedom. As he is speaking , you may notice the expression on his face that he is terrified but trying his very best to hide it on while on camera. He will also tell us that they are already moving troops into the surrounding area of Iran with the help of UK and France.
Phase #2One week after the presidents live speech , while we sit at home watching our normal television program, we are all suddenly interrupted with breaking news on a huge story. As fear creeps into our minds, we are told that the Grand Central Station in downtown New York was blown up by a mini nuke from a suspected Iranian terrorist. The toll death toll exceeds over $7,000 people including women and children. America is now as afraid as ever when a 2nd News update flashes across the screen and announces a 2nd terror attack has occurred. We are all shocked when it is announced that 1/2 of California has been disintegrated by a huge nuclear bomb. Panic starts to run nation wide as people start to flood he streets in chaos, breaking into cars, stores and vandalizing everything they see in their way.
Phase #3President Obama quickly addresses the world we are officially at war with Iran and those who harbor them. He announces that we have just send over 15 nukes into the main capital to Iran in retaliation of the California and New York bombing. So far the US death toll is at 19,744,00 people including women and children. America is outraged and has already started looking for anyone who looks like an Iranian and immediately kills them and their families for the terror bombing on the 2 major cities.
Phase #4America has detonated over 15 Nukes over Iran's major cities which kills over 33,000,000 people and the number continues to grow. Russia (Iran's ally) and China and North Korea has joined the war on Iran's side against the USA. France, UK, and Japan are now in the war officially as well, aiding USA. We now have the superpowers of the world against each other. Russian sends Nukes into Israel , while China and North Korea wipe France off the map with their nukes, completely wiping them off the map. The whole world is now in complete panic as counties are sending each other nukes and wiping each other out.
Phase #5Nearly half of the world is wiped out at this point, leaving US major cities still standing and half of Iran's military gone. America reveals a super secret weapon they have been working secretly on for about 2 decades. They send a missile that is not only undetectable, but can reach as far as the middle east and explodes in the air, releasing a super toxic biological micro organisms that spread into the air and kill anything that breathes it within seconds of inhaling it. This finishes Iran's military off within less than 24 hours after the missile has exploded over Iran's air space.
Now, I am not saying this is going to happened, but its just my own personal opinion on what might happened if USA went to war with Iran. I would love to get other theories and ideas on what might happened if this war broke out.