What was one of the key reasons for Confederation related to defense?

One of the key reasons for Confederation related to defense was indeed the threat of American expansionism. During the mid-19th century, there was widespread concern among the provinces that the United States might attempt to annex British North American territories. This fear was fueled by several factors, including the recent expansion of the U.S. territory, particularly after the American Civil War, and a sense of vulnerability due to the lack of a unified military defense against potential aggression.

The idea of Confederation emerged as a solution to this threat, proposing that the British North American colonies unite into a single political entity. This unification was seen as a necessary step to strengthen collective defense capabilities, ensuring that the provinces could better protect themselves against further territorial ambitions from the U.S. By forming a confederation, these provinces aimed to pool resources and military forces, creating a more robust defense system.

Other factors like the need for naval superiority, protection from Indigenous peoples, and concerns over Britain's military support were relevant but did not carry the same weight in prompting the movement toward Confederation compared to the imminent threat posed by American expansionism.