Everyone's obviously going to focus on how Canada could turn the screws on US agriculture by messing with our potash supply, or timber, etc, but what if this just shortcuts straight to an oil/energy embargo?
Nothing is going to shut down this tomfoolery quicker than mass civil unrest as gas prices shoot up over $10 a gallon. It might be possible to counter this temporarily with the petroleum reserves but even that will take a while to get going.