From: Utopia Planitia Fleet Yards
From the manual:
Infantry Units – There are up to 4 different types of infantry units depending on the country: divisions, small corps, large corps, and armies. All infantry corps and armies generally have the same attributes and function in the same way. Some countries have slightly different factors for their own infantry class units. Instead of generating a large number of infantry units with different minor purposes WarPlan allows players to detach smaller units from larger ones to cover garrison duty or activity behind the line while assigning specific advancements and availability of reinforcements.
Infantry units are the only units that may be divided and split up. Large and small corps can be merged or split off into small corps and divisions. Armies can be split into two smaller armies. A division can merge with a division or a small corps to make a small or large corps. A small infantry army can merge with another small infantry army to create a large infantry army. The merging unit will adapt the advancement of the unit into which it is merging. The lower of the two advancement levels will be the new advancement level of the merged unit. Some countries, such as Italy, can’t split or merge due to the small size of their historical division and inherit lack
of combat effectiveness as a single division within the scale of the game. Small corps can’t merge into a division.
Infantry Division – 10 strength, can form a small corps with another division or a large corps with a small corps
Infantry Corps Small – 20 strength, can form a large corps with a division or split into 2 divisions
Infantry Corps Large – 30 strength, can be split into a small corps and a division
Infantry Army – 15 to 36 strength, can be split in half forming 2 smaller armies of 7–18 strength each
The only major power to have army units is Russia
Chancellor Gorkon to Captain James T. Kirk:
You don't trust me, do you? I don't blame you. If there is to be a brave new world, our generation is going to have the hardest time living in it.