The U.S. is fine. All they have to do is play the role of "bouncer" for Israel and ensure that no other conventional military joins the conflict. They're going to pass a $14.8 billion usd emergency military aid bill for Israel, which, of course, is pennies for them overall when we're talking military spending. Meanwhile, they'll pass a much larger spending bill for Ukraine, designed to continue to bleed the Soviet stockpile dry. They have no interest in Russia losing the war in Ukraine, quickly, or in any substantial way. They simply want to get a cheap price to eliminate the Russian stocpile of Soviet weapons, for an extremely cheap price, sending mostly weapons that have been stockpiled in wearhouses that were meant for the Cold War. Meanwhile, they got NATO expansion in the form of Finland and Sweden is right behind them. That means, the entire Russian border to West, with the exception of Belarus (an irrelevant country) will be protected by Article 5, putting our Russian adversaries in a terrible position relative to their expansionist hopes.
USA has no real interest in bombing Iran back into the stone age. A destabilized Iran (the mistake they made in Iraq) does nothing to support their national interests. Instead, they'll simply focus their efforts on using their military toys to hunt Iranian proxies in the region, and keep any significant force from joining the fight in any meaningful way against Israel.
It costs them very little to drone strike some proxies, or launch a couple cruise missiles into Syria, Lebanon, etc, etc just to let the proxy groups in the region know they're watching.
