The Cascadian invasion of the Alaskan Union occurred on 21 and 22 December, 2020, in which the armed forces of Cascadia mounted a full scale invasion of the Alaskan Union following a territorial dispute. Cascadian forces quickly overwhelmed any armed resistance by the Alaskan Union and were able to secure and control a vast majority of the AU's territory and infrastructure, including roads and ice highways.
The Republic of Cascadia had been engaged in a large scale game of geopolitical chess over its sphere of influence over Alaska for a number of months prior to the outbreak of war, having been admitting multiple new provinces and constantly working on border agreements with non-Cascadian polities in the region. The most recent point of contention between Cascadia and an Alaskan adversary, possibly setting the precedent for further military involvement in Alaska was during the brief war with the Kidarite Kingdom over territorial control. The foundation of the Alaskan Union in the fall of 2020 led to further pains in the region with Cascadia's attempts to maintain control over the territories of Alaska.
In mid December, 2020, the Alaskan Union began to grow into areas that were previously claimed by Cascadia. The town in particular that prompted the war proposal was Blood Fort, AU, which was created inside Cascadian territory. This prompted a proposal to be made in the Cascadian Senate to enter into a state of war with the Alaskan Union. The proposal passed late on 21 December 2020 and an armed force was immediately mobilized to begin the task of going to war with the Alaskan Union.
Early on 22 December 2020, a large force of Cascadian troops, supported by troops from one of its provincial militias along with foreign volunteers mounted an invasion of the Alaskan Union's territory in the central interior of Alaska. Troops moved up from a landing point along the southern Alaskan coast and moved through mountainous terrain to move to secure territory within the AU, starting with the occupation of Blood Fort. Military movements followed and secured critical infrastructure including an ice highway before the occupation of Cartier Landing, where Cascadian troops engaged AU militiamen in light combat. Once the area was secured, troops regrouped and pushed through unopposed to Pinecliff, and then onto the capital of Aureum, where Cascadian soldiers controlled the city before withdrawing outside of claims to form a siege. Aureum was the site of lava and PVP skirmishes, along with the first use of artillery bombardment by the Haida Defence Force in combat. The bulk of all casualties in the invasion occurred just outside of Aureum's city limits. During the siege of Aureum, all of the Alaskan Union's territory was effectively occupied by Cascadian forces, as no AU citizen was able to leave the besieged capital until Cascadian troops withdrew for the night.
The invasion left a disputed number of people dead. The official figures reported by Cascadia and the AU vary. It was reported by the Cascadian Armed Forces that 11 AU citizens were killed in action during the invasion, and that 1 Cascadian soldier died in combat. The figures reported by the Alaskan Union were that 6 AU citizens were killed, along with 1 Cascadian. The siege started by Cascadia was abandoned by 2AM on 22 December.