What event is followed by Rab entering the war? What are Johnny's feelings about this?

Rab ultimately does die.

Rab sneaks out of Boston to go to Lexington and fight with the militia there. Johnny is worried that Rab is too eager to put himself in a dangerous situation. Yes you are correct, Rab is killed. Johnny learns that Rab was shot in the first volley of bullets. He visits the dying Rab at a tavern, where he is attended by Dr. Warren. The young men share a warm moment of friendship. Rab thanks Johnny for the musket and is only saddened that he couldn’t use it before he was wounded.By this point in the story, Johnny has come full circle. He no longer shies away from physical pain or work, as he did in the early chapters set at the Laphams’ house. He is also able to think rationally about his behavior rather than surrendering to his first impulses; he is stoic about Rab’s death and agrees to the operation because he knows that doing these things will be better in the long run, even if they are difficult now.