In the last few quests leading up to the activation of Project Purity, the Lone Wanderer is faced with a number of decisions, but one stands out above the rest:
Contaminating or not contaminating Project Purity with the modified FEV.
Which of these two options represent the canonical timeline of the Fallout games as a whole?
Best Answer
As far as I know, there is no canonical version of events (yet). Only two games take place after the events of Fallout 3:
The Broken Steel add-on, which contains eventualities for both cases of the choice.
Fallout: New Vegas, which takes place in a very distant location which should not be affected by that choice - in Fallout 3 it's stated pretty explicitly that the effects of the choice will take place in the Capital Wasteland, but no other region is mentioned.