[denethorthefirst]

I think it's very likely that Sauron would try to apply some sort of "scorched earth" policy in the event of an invasion. But he is far, far less powerful than Morgoth. He can't change or damage the geography like his master, he simply is not powerful enough. All the damage that he could inflict would be localized, superficial and relatively temporary (polluted rivers, destroyed forests and the like, maybe attempted genocide). I also think it's very likely that the actual military engagements in the event of an invasion would be rather limited. I guess that the morale of his troops would not be very high (at least after the first crushing defeat): how would his human/orc armies react to the splendor and magnificence of an obviously technically superior highly motivated, experienced and trained high-elven army? His armies deserted him before (after ar-pharazon invaded his empire, although that may have been a ruse ...) and it may happen again. Think of it like the Middle-Earth version of the 2003 invasion of Iraq: his armies put up some incompetent token resistance at first and then either flee or mass surrender, Sauron panicks and decides to hide in the East in some cave but a Maiar task force finds him and drags him out by his beard like Saddam Hussein. The whole conflict may only last a few weeks at most.