What kind of combat?
In the fields of Europe and ridges of Korea, the M1 Rifle was unsurpassed. In the jungle type warfare of Viet Nam, the M14 seemed good.
For the soldier, the shoulder fired rifle beats the handgun all hollow. the quad-fifty, the 105mm Howitzer, these are superb military weapons.
So, for a military sidearm, maybe a Colt Single Action Army, M1917 Colt or Smith & Wesson, or, if it has to be an auto loader, the M1911A1.
If I only had a sidearm in a combat situation, times would be dire indeed.
As a backup weapon in combat, I would want a pistol that weighs no more than 2.5 lbs loaded. Furthermore, I would want one I have been training with regularly to the point where I rely more on muscle memory than sight picture.
Without question, it must be utterly reliable, tolerant of submersion in salt water, mud and sand, consistently delivering COM hits out to 25yds.
Initially, I would want to be loaded with fresh subsonic JHP rounds to reduce my acoustic signature (a can is for assasination, the tradeoff of weight and size is not worth it for a backup weap) and moly coat the bullets in advance. The pistol should be also tolerant of dirty ammo and of a caliber compatible with those in use by the enemy. If I can't scavenge a proper battle rifle from a casualty or enemy, I want their ammo. Any and all jams should be able to be cleared without use of tools and not cause lasting damage to the weapon.
To narrow it down any more than that you need to select an enemy: Haji, Kim Il, Boris, etc.
Not that soldiers (except a few secret squirrel ninja types) can select their own pistols, anyway. And as several people have pointed out, pistols are inconsequential battlefield weapons, if we're talking about military combat.
Regarding the 5.7mm round as a good pistol choice against an armored "conventional army," it's pointless. My personal body armor will defeat 7.62X51mm AP, never mind some little hotrodded .22. With a pistol versus opponents in hard armor, it's face shots or nothing, so you're just as well off with the common 9mm as anything else. Most damage to armored combatants is done with explosives rather than small arms, anyway.
I would not choose any revolver for military purposes. And veering slightly off topic, while I like my "power plinker" Garand a great deal, I would vastly prefer my little M4 for real combat in a military context.
I think my ideal combat pistol would be the one that follows Murphy's Law 100%, meaning that since I am prepared for a fight, a fight will never happen.
Closest i've seen is something like the HK P30 or HK45 which have adjustable grips. Maybe even the M&P but then there are other things people are picky about. *shrug*
I say find one that is comfortable and then try it. If it shoots well then you found your gun
If you're looking for a great weapon straight out of the box that doesn't require much modification pick up a Sig at the shop and run some rounds through it.
I love my P229r SCT. It comes standard with a TruGlo tritium fiber optic front sight, Siglite rear sight, four 17 rounds mags for the 9mm version (14 rounds mags for the .40), feels great in my hand, and shoots far more accurately then I'll ever be able to shoot it. Plus in 9mm it's cheap, easy, and fun to shoot.