This is an important question and it’s impossible to predict the response of the US in this situation. It would depend on the nature of the Iranian and Hezbollah involvement in the fighting. The US has made clear its opposition to both Iran and Hezbollah in the region and has condemned Iran’s support of terrorism. The US has also condemned Hamas in the past and it is likely that US involvement would be more intense if the conflict escalates with Iran and Hezbollah joining the battle. A military response from the US is highly unlikely, however the US may act politically or diplomatically to support the interests of its allies in the region.