In "On the Rainy River," what position has Tim had about war in general since college?
Enotes only permits you to ask one question at a time, and therefore I have edited your question and will answer your first question alone. In "On the Rainy River" which is one of the short stories that make up the collection The Things They Carried, the narrator tells us how he almost fled to Canada to escape being drafted. In the second paragraph of the story we are told why the narrator made this escape attempt:
In June of 1968, a month after graduating from Macalester College, I was drafted to fight a war I hated. I was twenty-one years old. Young, yes, and politically naive, but even so the American war in Vietnam seems to me wrong. Certain blood was being shed for uncertain reasons. I saw no unity of purpose, no consensus on matters of philosophy or history or law.
Thus we can see that the narrator, although he himself says he was politically naive, could not agree with war. He "hated" war and in particular could find no reason to fight the Vietnam War he was being drafted to fight. He could understand none of the political necessity behind it, and instead saw lives like his being wasted for uncertain political reasoning.