The Capitol Building

Before the Great War, the United States Capitol housed the legislative branch of the government of the United States. It is located in Washington, D.C., on top of Capitol Hill at the east end of the Capital Mall. The other side of the Capitol opens onto Seward Square.

Sections
The building is broken into a number of areas. The most common way to enter the building in Fallout 3 is from the West side of the building from the mall, and this takes you to the appropriately-named "Capitol Building West Entrance". One can also explore the "Capitol Building East Entrance", the "Conference Room", and the "Hall of Columns". The dome, although centrally located, is considered part of the "Capitol Building West Entrance". Outside the capital building is a spawning spot for the enclave who will sometimes have a deathclaw

The battle for the Capitol Building
The Capitol serves as the battleground for a major conflict between the Super Mutants and the Talon Company.

Outside, from both the left and right each(!) three Talon Mercenaries and one Mr. Gutsy (so 6 Mercs, 2 Gutsies in total) will attack the stairs. A number of four Super Mutants, mainly Brutes (depends on player level though) will defend the Capitol. Two Mutants carry Missile Launchers, another one is equipped with a Minigun. There's a Master with a Chinese Assault Rifle higher up.

The Talons are sure to win the battle (even without losses), alone due to the plasma-weaponry on the Gutsies and the Mutants' bad aim with Missile Launchers.

An Enclave Vertibird and kill team (with an Enclave-controlled Deathclaw) may be encountered on the steps of the Mall side of the building, while the dome of the building hosts a Super Mutant Behemoth (see Super Mutant Behemoth below).

Super Mutant Behemoth
The Super Mutant Behemoth can be found in the Capitol Rotunda. There are two ways to go about killing him.

If you just want him dead, the smartest move would be to enter the hallway next to the entrance to the dome area, this should lead you to a terminal you can hack to activate the turret system. Remember to deactivate the turrets, as they will be hostile to everything in the room. After he is taken down, you can loot the various Mercs and Mutants for a large amount of ammo. There is also a large cache of missiles you can access once the room is clear. Even without hacking the turret, if the player stays completely out of the battle, the Talon Mercs may defeat the Behemoth on their own, due to superior numbers and the fact that one of them may carry a Fat Man. If the merc drops the Fat Man, be sure to beat your companion to the weapon. Companions do not take the blast radius of the Fat Man into account, often with disastrous results.

If you wish to take him down yourself for experience, be wary of the Mercs. As stated before, one comes equipped with a Fat Man, so it would be wise to enter sneaking mode and quickly take him out. At that point, he should go down with one or two Mini Nukes or a few missiles. There is also a Super Mutant with a Minigun on a ledge in the back of the room. If you are low on ammo, there is a surprising amount on the corpse of the Behemoth.

Capitol Building West Entrance

 * Nikola Tesla and You on a desk behind a {hard} locked door
 * U.S. Army: 30 Handy Flamethrower Recipes on a table, southeast part of the Capitol dome
 * Mini Nukes and a Fat Man on opponents and/or on shelves inside the dome

Capitol Building East Entrance

 * Tales of a Junktown Jerky Vendor on the middle desk in a row of three cubicles
 * Pre-War Book on a fallen cabinet next to that desk
 * Nuka-Cola Quantum a room over to the east, behind a coffee maker
 * Stimpaks on the cupboard opposite the Nuka-Cola Quantum

Conference Hall

 * Lying, Congressional Style appropriately stashed on a podium.

Hall of Columns

 * D.C. Journal of Internal Medicine on the upper level (a Talon Company sniper is stationed on a bridge, and the book is on a railing)
 * Chinese Army: Special Ops Training Manual behind a {hard} locked door (accessed from a small hallway found near a turret and a locked turret control terminal in the middle of the hall)

Gallery and concept art
On May 8, 2007, Bethesda Softworks released a piece of Fallout 3 concept art by Craig Mullins depicting the Capitol after the Great War, in a largely intact state.