basically we could have. not left them alone completely, but reduced it to aerial bombardment to keep their capabilities subdued for as long as we'd liked or until they surrendered.
by that point in the war the outcome was no longer in question, the whole issue then became the 'race for berlin' just to see who would get control over rebuilding germany into what each side wanted, not out of any strategic necessity.
the soviets invaded and took control because they wanted to spread their communism shit there too just like they had 'liberated' and set up 'friendly regimes' in poland and eastern europe. it was in their own interest to spread communism as far into europe as they could, it would keep an old enemy (germany) in check and would make russia more powerful.
the only reason the allies needed to do a full scale invasion of germany was to counter the 'soviet threat' and thus try to keep germany 'western' in order to help provide a buffer in europe against the red beast. the marshall plan wasnt done out of the kindness of our hearts
it was because we wanted germany there to counter soviet influence and for our own economic interests.
the goal of making war on a belligerent nation is to destroy their ability to continue to make war, and to remove them from illicitly aquired foreign territory. after this enemy has been removed from all foreign territories and their ability to continue warfare has been smashed and removed, then what threat are they? once you have owned them thoroughly and demonstrate that there is no way for them to do anything at all without your permission, what is the strategic purpose of invading their country? you already hold sway over them, they know it as well as you do, and formal surrender is inevitable. unless you are attempting to install your own government like the soviets for your own political and economic agendas, then there is no reason to waste an incredible amount of lives and resources by invading an already militarily defeated nation.
