The US Pentagon has completed a two-week “classified” war game in which it attempted to predict the outcome of an Israeli-Iranian war which resulted in US involvement. The first link gives the results of that war game exercise. The article states that specified US casualties would be minimal; limited to 200 dead on a US Navy ship hit by Iranian cruise missiles. However, that may understate the actual war game result as the article also states that a Pentagon General believed “an Israeli first strike would be likely to have dire consequences across the region and for the United States forces there.” The article also acknowledges the possibility of US losses due to Iranian proxies detonating car bombs in world capitals or heightened attacks vs. US forces in Afghanistan. The war game reportedly came to the very optimistic conclusion that the Iranians would make only a minimal and limited military response to any Israeli attack.
The war game assumed that an Israeli first strike against Iran followed by a US second strike (triggered by Iranian retaliation vs. the USA as well as Israel) would delay the Iranian nuclear program by three years. I think there are many people who would regard that as a reasonably good outcome.
Personally, I think that a minimal Iranian response to an attack vs. its nuclear facilities, while possible, is not likely. I think it is more likely that Iran would seize upon the attack as a reason to attack both Israeli and American targets. The war game apparently almost blithely dismisses the serious ramifications of the attacks vs. western powers via Iranian “proxies.” Hezbollah is an Iranian proxy. If it is ordered to unleash tens of thousands of rockets against Israel, the ramifications could get very ugly very fast as I believe the Netanyahu government will attack Hezbollah’s forces with the goal of annihilating them in Southern Lebanon, regardless of the civilian casualties inflicted by such a war. If Hezbollah launches rocket attacks to kill Israeli citizens, why should Israel have any concern for the casualties inflicted by an Israeli counterattack against Hezbollah missile bases in Southern Lebanon? Another Iranian proxy is Syria’s leader, Assad, who is under severe internal pressure to resign or go into exile. He might decide that he could stay in power by rallying his people behind him by joining a Hezbollah attack against Israel. Hezbollah sleeper cells in American and Europe could also cause many casualties on the home soil of western nations. If the Pentagon report did any detailed analysis of these possibilities, there is no mention of its conclusions. Iran could also retaliate by launching hundreds of cruise missiles against US bases in Afghanistan. Does the USA have anti-missile batteries at all US Afghan bases in case this happens? If not, and Iranian missiles hit US bases, military career heads will roll as this was an obvious step which should have been taken.
I wonder if the Pentagon report included the very real possibility that China or Russia has secretly placed their latest anti-ship, supersonic cruise missiles along Iran’s Persian Gulf coast so they can be tested against US Navy ship defenses in real wartime conditions. If these attacks were successful in sinking or heavily damaging US ships, Russia and China would be content to let Iran claim that Iranian missiles did the damage even though Iran, Russia and China would all know better. 
The second link cites Iran’s leader as saying Iran would “retaliate against any attack by Israeli or American forces on the same level (emphasis added).” That sounds like a pretty belligerent declaration. The article in the first link indicates the Pentagon Wargamers assume that Iran’s leader is mostly bluffing in his belligerent statements. I hope they are right, but the US military needs to prepare for the worst-case scenarios as well.
The allied leaders before World War II all assumed that Hitler was bluffing during his military build-up. He wasn’t bluffing as the world later realized. Hitler had every intention of using the weaponry he was building to wage a large-scale war. Ezekiel 38-39 warns us that Russia, China, Iran, and their anti-western allies aren’t bluffing in their current military build-ups in the latter days of our age. They have every intention of using them in an attack vs. the West and its allies at the end of our age. Indeed, Ezekiel 38-39 warns that such an attack is a certainty (see my article, What Ezekiel 38-39 Warn about a Future World War III [PDF]). That being the case, they would love to test their latest weapons against US military defenses in a limited, regional war. An Israeli/American war vs. Iran could give them that opportunity.