Iran has some significant logistical roadblocks to attack Israel. They can send their political puppet, Hezbollah, directly at the Israeli border from Lebanon, but the Iranian army itself has to pass through a combination of countries to directly bear arms on Israeli soil, like Iraq, Turkey (a NATO ally), Syria, Jordan, and/or Lebanon. None of these countries are going to be friendly to that action.
The vast majority of the Iranian air force are 30+ year old MIG 29s and F-14s (yes, the Top Gun plane). They have no F16s or F18s, and less than 10 modern air superiority fighter craft. They still rely heavily on the F4 Phantom, a fighter we retired almost 30 years ago. So Iran cannot control the skies in hostile territory, which means the Iranian columns would look like the Iraqi column fleeing Kuwait on the Highway of Death.
America has two carrier task groups in the region, plus the ability to move other assets to the area pretty quickly if shooting starts. There was a guy from England who tracked a bunch of F35s and their refuelers leaving Lakenheath in England, on their way to (I think) Italy, this past weekend.
So an all-out war seems unlikely, leaving terrorism, likely in the form of rockets, as the most likely option. And while that will pose significant risks to Israel, conventional rockets won't do much damage. The Iron Dome will take care of dozens, if not hundreds, of rockets. Those that get through will cause damage, but not enough to truly harm them. Even if Iran could ally with, say, Syria, Israel has shown their ability to not only defend, but defeat attacks from multiple countries at once.
The scary thing is, we know Iran is nearing nuclear capabilities. They don't have to rocket that over to Israel, and shouldn't, due to the likelihood of it being shot down.
But what if they infiltrate Israel with a dirty bomb? Not truly nuclear, but enough radioactive material to cause widespread death and destruction? That's why I think the US overtly announced the movement of a missile submarine to the area. We don't usually talk about the whereabouts of our nuclear subs, but we overtly announced that over the weekend.