What did president jefferson mean by "a mere matter of marching" and why was he proved wrong

Jefferson referred to the acquisition of Canada as a "mere matter of marching." What he meant is that the United States would be able to take over control of Canada by simply sending soldiers to invade this country. He was proven wrong on this manner for several reasons.

1) Canadian terrain- Canada's harsh winter and climate made this task extremely difficult to accomplish.

2) Effectiveness of the British army- At this time, the British army was still in Canada as this was still their territory. The British also wanted to make sure that they still had a significant presence in the North American continent. So, with all of this in mind, the British were ready to fight off the American military that was bound to invade.