You're walking, crouched, inside the enemy entrenchment, shouting back every few seconds, just like you're shooting forward every few sec's. If you stop your shouting the man behind you may well turn the corner shooting - your shouting is your only way of letting him know you're still alive - he can't tell your shots from all the shooting and explosions around. If he kept constant eye-contact with you, you could both be killed by the same grenade or burst of fire. You can't lift your head, either - someone, an enemy unit or one of yours that doesn't know your squad has reached this entrenchment is bombarding the place - probably heavy mortars and not artillery, you guess. You can hardly hear yourself think, hardly feel the pain in your feet, and you haven't taken your boots off in three days. You fumble for a second as you take a fresh magazine and stick it in your M16. If you encounter another squad from your unit you'll be lucky if you're not all dead before you discover the mistake you've made.

What I described is probably one of the most "intimate" forms of combat in modern warfare. Mostly, people are just sitting in tanks, fixed-wing aircraft, helicopters, etc. and shooting each other to death by remote control. During the cold war, if the world would have been destroyed due to some stupid policy or stupid mistake, the "combatants" would never have seen each other. Modern warfare has, to a large extent, "dehumanized" warfare. The rising casualty count in wars isn't only due to us being able to kill more people faster - it's because of the loss of the means of communication futurebird mentioned. It's very hard for one person to surrender during war today. Of-course, low intensity conflict can be different, and obviously surrender is also an option (sometimes) for small units left stuck behind enemy lines, downed pilots, surrounded forces, etc. Modern warfare has negated the posture and submit reflexes, and with them has ruined many of the violence regulating behaviors we humans probably used to have. If you were going to stab someone to death with your sword, you could see the fear in his eyes, watch him drop his weapon, surrender. When you've locked a sidewinder on the enemy airplane's tail, you can hardly tell how young the pilot is, and what a terrible waste of life this whole bombing mission you're escorting is.

On the other hand, It's worth bearing in mind, that submission (unlike retreat) isn't all that common in the chimp world, for example, between members of different groups. Chimpanzee "patrols" will kill a foreign male encountered if they have the chance. Submissive behavior is mostly left for inter-group communication.