This question refers to "On the Rainy River," which is part of The Things They Carried by Tim O'Brien.
In this story, the speaker describes how he reacted when he got his draft notice at the age of 21. The speaker tells us that, before this event, being against the Vietnam war in college was "entirely an intellectual activity." What the speaker means is that he did not actually take any concrete actions of opposition. Instead, the war was only present in his mind as a situation that he intellectually understood, but that was very far from the reality of his life and that did not concern him greatly.