How does o'brien create tension and suspense leading up to the climax of this story?

This chapter is pivotal, as Tim must choose dodging the draft and going to Canada or staying in America and being drafted to go to Vietnam. Tim is physically on the border between both countries on that rainy day on the river. Tim decides that choosing Canada and being estranged from his family would not be freedom. His decision sets up the rest of the story, mostly in Vietnam, that leads to the climax.