This is one of the reasons they went to war with the Enclave, apart from the imminent threat to their existance of course.
This is true for Fallout 3.
In Fallout 2 however, they simply cowered in their bunker and let somone else handle it.
I give this to the East Coast BOS, at least they had the courage to meet the Enclave head on. Not so much at the purifier (since you know...the big Deus-ex machinima that is Liberty Prime did all the work) but I did have
some respect for their suicidal mission at AAFB. Charging shock troops in Hellfire armor while being bombared with deadly arial assaults and flaming napalm (and being massacred as a result of it) in order to buy some time for the LW takes some guts. Its almost on part with the Enclave soldiers who fought against Liberty Prime.