Of course it will be more sanctions, more pressure to other countries to denounce Iran, more proxy war with funding literal terrorist groups and telling them to hit whoever the US doesn't like, also probably bomb a few Iranian assests because what actually can they do?
An open war rather requires justified claims. It is gruesome and the horrible results are directly tied to the war. A combination of sinister proxy war, subterfuge, coercion and forced poverty are slow killers and will have way fewer dissidents. It is the name of the game for half of the US oppression policing on the world.