But you can shove 4-million tanks onto a single square in Civ4. That's the stupid part.
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In the real world (WW2 and older battles anyway), soldiers spread out, trying to out-flank each other and gain terrain advantages. You can't just "stack" your entire army on top of itself. You have to deal with terrain, chokepoints, and movement logistics.
"The battle of the Bulge" was a thing, because terrain and battle-line formations matter in the real world. (Well... at least historically. I'm not sure if it matters as much with today's jet fighters and super-carriers). The USA lost tens of thousands of troops, because holding the line was incredibly important. (And the Germans similarly sacrificed tens of thousands of troops in the hope that they get a breakout offensive)
Except movement-logistics are boring and tedious (see Civ5 / Civ6 endgames). Allow unrealistic stacking but remove any tactical advantage from it (ie: Civ2), so that the game is a bit more fun to play.
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Anyone playing Civ4 will inevitably draw their units together into a singular death-stack. The entire concept of "fronts" is almost completely devoid in Civ4.