Therefore, the structure of some troops can only be streamlined: without the Heavy Artillery Group, there's no way, then cancel it, and use a Mountain Artillery Regiment as a makeshift.
At this stage, even the Mountain Artillery Regiment is unavailable, so use the Mortar Battalion as a substitute. Some troops don't even have proper artillery and are directly sent to the front line as cannon fodder.
If there aren't any artillery units, use Gun Grenades as replacements; if there aren't any Anti-Tank Guns, use Anti-Tank Rocket Launchers as makeshift; if there are no Anti-Aircraft Guns, give the soldiers two High-Angle Machine Guns to bolster courage...
Such troop structures are naturally reduced, especially the commanders, which can be significantly decreased: you have to assign officers to the Anti-Tank Gun units, and these officers need relevant expertise, need to understand military maps, need a foundation in surveying, and leadership skills.
