Many Regiments and Brigades serving in the XXII Corps were only temporarily assigned to it. Some mainly served during times when they were reconstituting due to battle casualties, while others were trained in the vicinity of Washington before going into the field. Yet others were heavy artillery regiments assigned to the fortifications surrounding the capital. Many units, including heavy artillery regiments, left when more soldiers were needed during Grant's Overland Campaign and continued through the Richmond-Petersburg Campaign until the end of the war. Some units began serving before the XXII Corps was formed.