There are actually several metaphors in Sam Roberts's song “An American Draft Dodger in Thunder Bay.” The song centers around a young man who is opposed to fighting in the Vietnam War, yet he is drafted (or at least afraid of being drafted). So against his father's wishes, he goes to Canada to escape the draft, and he settles into a new life there.
Life has been good in the young man's hometown. In fact, it is “postcard living,” a metaphor that suggests comfort and prosperity, at least on the surface. But then “Vietnam moved next door.” Here is another metaphor. Vietnam didn't literally move next door, but it moved into a front-and-center position in the lives of the townspeople. They became intimately involved in the war.
The young man takes off to Canada, for he does not want to fight. The going is “high stakes.” This metaphor shows that the young man is taking a risk in leaving the country and dodging the draft. It is a gamble, and if it fails, he could lose much, like his freedom.
The young man is from Mississippi, so the Canadian cold affects him negatively. This cold may also be a metaphor of sorts, suggesting the chill the young man feels from his neighbors and even his father because of his resistance to the war. Perhaps the physical cold of Canada is easier to bear than the emotional cold of his loved ones.
The young man goes to live on the Canadian Shield. This is an actual area in Ontario, but it is also a metaphor. Canada shields, or protects, him from the American draft. He puts down roots there, even though the fields are frozen, another metaphor for settling in for a long stay. He also learns the game, a metaphor for learning how to live in a new place.
Indeed, the young man has crossed the line, literally and figuratively. He has crossed the border into Canada, and he has crossed the line of disobedience to the American government. There is no going back now.