I like to run campaigns, probably at Div level, on the map to generate interesting scenarios, then play encounters at battlegroup level on the table.
In Moderns, I used to use the Challenger rules, which were pretty clunky and tried to cover every possible encounter between 1950 and 2000. BUT, the ground scale at 1:300 was the same as the model scale, which looks good and works well on the table.
It does make it difficult to put much more than a battlegroup on the table at any one time, as the table is rarely more than 3km square (if you want to pack more troops into that area, then I've got a Tacnuc with your name on it, opps sorry, wrong period).
All armies can be divided into two parts:
2. Support arms.