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Who bought this debt (US treasury bonds)? Mostly China, Japan, North Americans and Europeans … So they depend on the system, the circle closes. The US is the biggest importer in the world, and the years since 1945 where used for exporter nations to optimize and streamline for the American costumer. Breaking away from that system would be painful.
Say you are a oil producer, and frack gas and oil in Texas, an industry that supported trump. Now imagine what tariffs on oil-imports from Canada means for your business: rising prices, since gas from Canada would be less competitive. Same with logging and wood products.