Stanislao explained about the fuel. My personal advice is not to take information from Fallout 3, especially those about Enclave as canon. Fallout 3 is a synonym for an illogicality at its best. As for people in Navarro, it is true, there weren't many families there, but think about it like this. You have lost your HQ from your radar, some report that they have seen something what seems to be a nuclear explosion from the high seas, a flash seen in the direction of your HQ (we can presume somebody had seen a flash and informed the mainland base), you have no or very little communication, and you will soon run out from fuel for your vehicles. If those aren't reasons enough, some time later you get reports that massive army is coming your way, best case scenario - take the base, worst case scenario - annihilate the base. Now tell me, in these conditions and circumstances, how patriotic you would be? NCR-Brotherhood war had ended few years before the threat of the Legion arose, or at least the Mojave Chapter conflict ended at that time. As for the whole war itself, its exact length is unknown. NCR still suffered from that war and suffered greatly, which is evident in how they got stretched thin in the Mojave, with more soldiers redirected to fight the Legion, and all those caravan routes, settlements and trading hubs being left without any protection.