The final mission; Paladin Tristan asks you to enter the Enclave's base at the Adam's Air Force Base. Firstly, you'll need to make your way to the White House Plaza, from there, enter the Metro and fight your way through. At the end, you'll reach a train that you'll have to ride. After the ride, you'll end up at the base. Proceed through the base, and you'll eventually come it a Mobile Base Crawler. Check the terminals as you go, one of them gives you access to the Crawler. After finding the right one, enter the Crawler and make your way through, killing Enclave as you go. You'll eventually come to the Orbital Strike, activate it and watch the fireworks.