Worst case, world war three. If enough other countries back Iran, they might retaliate in a way that triggers something like the nato treaty and you get some combination of countries compelled to do something, which could set off a series of conflicts that might spread.

More likely, some skirmishes and back and forth attacks happen, but nobody outside the region becomes involved directly.

Or, Iran could just posture and use it as political leverage to strengthen their position with allies and the various blocs around the world.

Obviously, there’s variants of those, and plenty of really unlikely options. But based on how iran has acted in the past and how little anyone is pressuring Israel currently, it doesn’t seem like it will escalate unless something else changes