This is an interesting question. Humans certainly have a major impact on the environment. Wars can be particularly destructive, not only to our societies, but to our environment. Wars have been fought in different regions by many different methods, so there are many different ways that they have affected the environment. I will highlight a profound example that comes to mind.

One example of the environmental impact of war is the use of a chemical known as agent orange in the Vietnam War. Agent orange was an herbicide mixture that was used to kill the vegetation of dense forests in Vietnam. This was intended to expose the Vietcong and North Vietnamese Army forces that used the thick jungles to conceal movement. It was also used to destroy food crops that the US and Republic of Vietnam forces thought were used to supply the “enemy”. The chemical mixture was sprayed over the jungles and paddies of Vietnam at a rate that was 10 times greater than it was used in the US for farming purposes (Stellman et al., 2003). Between 1961 and 1971, 2 million hectares (20%) of the forests of South Vietnam were cleared with agent orange (Westing, 1971). That is a very large area.

References
Stellman, J.M. et al., 2003. The extent and patterns of usage of Agent Orange and other herbicides in Vietnam. Nature, 422, 681–687.
Westing, A.H., 1971. Ecological effects of military defoliation on the forests of South Vietnam. BioScience, 21, 893–898.

I don't think anybody really knows, although it depends on the nature of the war. Wars in which a lot of indiscriminate bombing and use of toxic chemicals (e.g. World War I, the Vietnam War) cause significant damage. War at sea resulting in sinking ships releases a lot of oil onto the ocean. Small-scale wars probably don't matter so much. Nobody really knows what the effect of nuclear war would be on the environment, but lessons learned from Chernobyl tell us that radioactivity is far more dangerous to humans than it is to the environment itself.

The effect of war on the environment is dependent on what happens in the war. The social environment is the most beaten up, such as when whole communities are destroyed. Then all the destroyed homes, shops and other buildings have to be cleared away and new ones built.