Crusade

History
A crusade is the term used by the Empire for a large-scale military campaign organized in order to conquer or retake multiple planets in the galaxy. Crusades are usually very large-scale offensives that require a major mobilization of Imperial military resources across multiple planets and often are fought as a series of smaller campaigns across the chosen theatre of operations. Because of the sheer size and ambition of a crusade, it  requires the deployment of forces from more than one branch of the Imperial military, and so are usually led by a general  granted the special rank of Lord Crusader. An Imperial Crusade can only be declared by a decree by the Emperor and can include any Imperial campaign that is considered either of great strategic importance to the continued health of the Emperor's realm or an offensive unusually broad in scope that will include multiple worlds and last for several years. However, the emperor has also declared relatively short-lived offensives to be Crusades that were resolved with only a single battle when the objectives were considered to be of particular strategic or political importance.