In ancient warfare, the Greek hoplites deployed in tight formations called phalanges, which were eight or more ranks deep. The usefulness on the rear ranks is still a matter of scholarly debate, but many now believe that the rear ranks had a role beyond replenishing casualties in the front ranks. The "physical pushing match" school of thought believes that the rear ranks exerted so much pressure on the front ranks that the resulting shock action of the phalanx was, at least partially, a physical push forward.
I'm enjoying John Tiller's free demo for the Panzer Campaigns series. You can ask for it in this webpage. I recommend this fantastic graphics mod for a more complete enjoyment.
The demo features the Soviet offensive in the river Mius in July 1943. I'm not new to commanding a whole Soviet WWII front in an offensive against prepared German defenses. But I'm not getting any better!
The Soviets had a penchant for precise calculations of the attacker/defender ratios and as a result, many breakthroughs were concentrated in narrow spaces so to overwhelm the enemy. While playing this scenario today, I've got some of the infantry divisions tasked with the initial breakthrough entangled in bunker-busting and stopped cold in their tracks. The follow up units that were to exploit the breakthrough couldn't move forward for a hasty support (you can't stack that many units in a single hex).