Alberta was a province of Canada. In 1881, during the Second Mexican War, the Canadian Army in the west was concentrated in Alberta and placed under the leadership of Major General Charles Gordon. When the British finally increased their involvement in the war, the army crossed the border into Montana with the intention of raiding the gold mines there, but were routed and retreated back into Alberta.
In 1914, during the Great War, the province was invaded by the US Army, and in 1917, when Canada surrendered, the entire province was occupied by the United States.
Along with taking up all of OTL Alberta, Albertus also takes up OTL west-central Montana, excluding the westernmost tip, which is part of the province of Oregon.[3]