After finishing the quest Boulder City Showdown, Lieutenant Monroe will start walking to Camp Forlorn Hope. If he arrives there, he will walk to the campfire in the center of the camp and will disappear from the game after a while.
He only offers an unchanged dialogue tree which gives background information about Hoover Dam, President Kimball's inspection tour and the Boulder City incident in 2277.
Once at Camp Forlorn Hope, Monroe often engages in the defense of the base and may be killed.
When killing everyone in Camp Forlorn Hope, such as during the mission We Are Legion, and Monroe is there, he will be the only NCR soldier that will not become hostile towards the player character if another soldier is killed.