In Volume I, Part One, VI, Pierre and Prince Andrei are talking:
“Well, what makes you go to war?” asked Pierre.
“What makes me? I don’t know, I have to. Besides, I’m going . . .” He paused, “I’m going because this life I lead here, this life–is not for me!”
In Part II of The Pensees, “The Misery of Man Without God”, Pascal writes
One would not buy commissions in the army at such a price, if it were not found intolerable to remain in the city.
Which leads to the consideration that the revolution in Pierre’s character after his captivity perhaps comes from his discovering perforce “how to remain quietly in a room”, the inability to do which Pascal regards as the source of all human misfortunes.