U.S government nuked D.C area so Chinese would not capture it. Look at the nuke in Megaton and then go to Fort Constantine Bomb Storage.

Personally, I think that once you get the blaster, thats when the crashes start to happen more often. You know, for ammo XD. But, if you have ever been to the western part of the wastes, you will find many radio stations. If you turn them on, you can here some kind of coded message, that sounds alienish. And! You see those satellite dishes out there? Well when you type in a code for a certain one, then walk outside of it, you can see many rockets falling from the sky and onto the earth. Its weird, but cool.

Those coded messages lead to a number of secret places where dead humans are.

The nuclear payloads dropping from space are from orbiting weapons platforms that were not used...

nothing alien about either, sorry.

As for the moon, I think it's plausible that the Enclave began secret projects with Vault-Tec (Moon-Tec?!) to create military installations on the moon. Afterall, publically, moon missions ended shortly before 2077 -- but that leaves a lot of time for the development of moon colonies.

The existence of the Vaults themselves and the nature of many as experiments could have been experiments in long-term isolation -- which would have to be factored in on if the Enclave were planning on populating a lunar base. Or even an orbiting space station.

So what happened to the Enclave's plan? Was it generally abandoned when the oil rig was destroyed? In FO3 the Enclave seem to be pre-occupied with retaking control of D.C., but this easily could be for the same purposes. Afterall, this is much later and the Enclave have met some setbacks. Most of the Vaults have been opened and the survivors have re-populated the place.