I used to work with a guy, an American from upstate NY, who went to McGill University in Montreal. He used to tell me how surprised he was to hear the same widespread fears of US invasion among most Canadian students he knew there. Whether it's true or not, paranoid or not, it is a widely held belief among Canadians. Most Americans, including myself, are surprised by this.
Dan already pointed out some facts of history that are worth repeating.
- During the Revolutionary War, an American army really did invade Canada in 1775. It failed.
- In 1845 we took Texas which had rebelled against Mexico, but Mexico still claimed it. In 1846 we really did invade Mexico, taking large portions of land from them, including what would become the states of California, Nevada, Utah, most of Arizona, as well as portions of Wyoming, Colorado, and New Mexico.
- During the years preceding the US Civil War there were several abortive attempts by Southerners to capture and annex Cuba, Nicaragua and Honduras. Their goal was to annex them as slave-holding territories, with the intention of making them US slave states. Look up William Walker.
Although this was claimed at the time by some, it was far from reality. The USA had banned importing new slaves in 1808. But the international slave trade was banned by Great Britain in 1807, enforcing it in the Atlantic and Caribbean with the Royal Navy. Ever since then, slaves held within the southern US became increasingly valuable. So valuable, that slaves became more valuable than land in some cotton growing areas. The widespread use of US-born slaves in the southern states long after 1808, strongly argues against the idea that slaves were barely necessary because of the advances brought on by the industrial revolution.
In fact, growing cotton with slave labor is what prevented the south from entering and benefiting from the industrial age. Plantation owners would rather sell cotton as a raw material to the industrial north, as well as Great Britain and France. They would rather pocket the profits, than pay for developing any local cotton fabric weaving industry in the south.
So claiming that the industrial revolution had made slaves unnecessary is untrue. If anything, it is part of the false 'revisionist history', spread by the south in the decades after 1880, that claimed the Civil War was not about slavery, but about state's rights.