It's already been established that RED and BLU are the two largest corporations and are competing for global dominance. So the fight literally takes place across the world. However, many of the maps are located in the American midwest. The reason for this is that the original conflict started over the land Zephaniah Mann left to his two sons. It was a huge expanse of what seemed to be worthless land in the "new world". The two brothers thought vigorously over it and continue to do so, and though it is a global conflict it is not unsurprising that much of the fighting still takes place in the land they first squabbled over.
I just realized that TF2 has this elaborate backstory now when it literally used to just be two different colored teams blowing each other up.