When the Great War broke out in 1914, the Royal Canadian Navy was a relatively small force compared with their US counterpart, but they were still formidable, consisting of destroyers, cruisers, Great Lakes Battleships, and submersibles. Their prime task was not only defending their Great Lakes shorelines, but keeping the waters near Canada open for supply convoys that came from both the Pacific and Atlantic Oceans, but the Atlantic was the major priority.
As the whole North American continent mobilized for war, the Canadian Great Lakes Battleships left port. The opening phases of the war saw victory for the RCN on the Great Lakes. While the US Navy chose to pursue a fleet of Great Lakes Battleships that would travel up the coast proving fire support for the advancing US Army, the Canadians sought a more practical method of defending their Great Lakes coastline, by sowing the seas thick with sea mines. Along with the mines, Canadian submersibles also proved more than effective, sinking a Great Lakes Battleship and a couple of light cruisers. This brought the US Naval offensive to a halt. The RCN also scored another victory over their US rivals in the Pacific where they, the British and their Japanese allies managed to bottle up the Seattle squadron of the US Pacific Fleet, thus keeping the Pacific supply route open.
As the US Army began to approach on Toronto in 1917, the Canadian Great Lakes Battleships assisted in the city's defence, their guns pounded the US lines. Despite constant US air attacks, they kept up their support until Canada asked for an armistices. Although the RCN remained undefeated, they were forced to disband after the US annexed the country.