|The following is based on Fallout Tactics and some details might contradict canon.|
One night prior to the invasion, Brotherhood intelligence operatives were killed in a surprise ambush. The super mutants had become familiar with their covert methods.
What was left of the Brotherhood intelligence ops had discovered the location of one of the Super Mutant's staging areas; a semi-permanent military base in the rubble-strewn city of Jefferson. The scribes came to the conclusion that the facility was a weapons manufacturing plant.
The Warrior's team was sent in with two objectives: to render the base's power generators permanently inoperative, and to force the mutants into a rout.
The Warrior eventually discovered that the factory wasn't a weapon's manufacturing plant at all, but a research facility led by Erkal, who was trying to cure the super mutants' sterility problem. He or she either destroyed the research or left it for the Brotherhood to capture for use as a bargaining chip.
The Invasion of Jefferson appears only in Fallout Tactics.