The Capital building has a respawing Super Mutant vs. Talon Company battle outside(opposite side as the National Mall). Besides that the only real large scale battle is in the final mission, otherwise you will only see skirmishes with no more than 10 combatants in most cases.
If you are playing the PC version of Fallout 3, I suggest you take a look at Mart's Mutant Mod, found here:, which has an option for increasing spawns. The result is that DC can turn into a serious warzone with various factions engaging each other in large scale firefights. Recently I came across a large squad of Talon Company mercs fighting around 20 super mutants, 5 centaurs and a super mutant behemoth. The exchange of lead lasted about ten minutes and it got very, very ugly.