That said, we'd have to throw much bigger rocks to penetrate their atmosphere. And the likely (to me) actual plans would be:
Us: launch to the Moon, set up there, launch rocks from the Moon to Venus.
Venusians: launch and travel to the asteroid belt, launch an asteroid toward Earth.
The more I think about it, the more I realize that our plan would be the same as theirs: we'd both be heading for the asteroid belt, because nothing we could reasonably launch from the Moon would put a dent in Venus with that atmosphere.
And if we assume they actually can launch through that atmosphere, we're screwed: if they can do that, they're way ahead of us.
The situation I replied to assumed they were both inhabitable planets which I assume means Earth like atmosphere on Venus. The thick atmosphere complicates things, but I don't think you actually have to hit the ground. Tunguska didn't even get near the ground and it still leveled 1000 km^2. Also if Venus has the atmosphere there's no point of a war since there's no benefit to conquest in either direction.
