There was a threat of American expansion into British North America. Where were the concern of the Americans expanding too? Why would this area be a prime target?
The Americans wanted to expand in Canada's southwest. The rallying cry was "54-40 or Fight!" This is the latitude that is far north of the present 49 degree boundary.
http://en.wikipedia.org/wiki/Oregon_boundary_dispute
The concern of American expansion into British North America primarily revolved around two key areas: Upper Canada (present-day Ontario) and the Oregon Country.
Upper Canada was of particular concern to the Americans due to its strategic location and resources. Control over Upper Canada would have provided the Americans with access to important waterways such as the Great Lakes and the St. Lawrence River, allowing for greater trade and transportation routes. Additionally, Upper Canada was rich in natural resources like timber, fur, and agricultural land, making it an attractive target for American expansion.
The Oregon Country, located in the Pacific Northwest region of North America, was also a prime target for American expansion. This area, encompassing what is now Oregon, Washington, and parts of British Columbia, had significant economic potential. It had fertile soil for agriculture, valuable fur-bearing animals, and opportunities for trade with Asia via the Pacific Ocean. The Americans sought to claim the Oregon Country to expand their sphere of influence and secure valuable resources and trade routes.
It is worth noting that the concern over American expansion mainly stemmed from British perspective during the 19th century. Americans, on the other hand, saw their westward expansion as a natural and manifest destiny.