You may've seen most of these before, but thought people might like to
see em again.
There are no atheists in foxholes.
Those who beat their swords into plowshares will plow for those who don't. 
Tracers work both ways - US Army Ordnance
Try & look important, they may be low on ammo - Infantry Journal
When in doubt, empty the magazine.
When you're short of everything but the enemy, you're in combat.
When the pin is pulled, Mr. Grenade is NOT our friend - Us Marines Corp
We are not retreating, we are advancing in another direction.
A slipping gear could let your grenade launcher fire when you least expect it.
That'd make you quite unpopular in what's left of your unit.
Don't look conspicuous, it draws fire.
5 second fuses only last 3 seconds - Infantry Journal
If the enemy is in range, so are you - Infantry Journal
If your attack is going too well, you've walked into an ambush - Infantry Journal
Look at an Infantryman's eyes and you can tell how much war he's seen
- Bill Mauldin 1944
It is generally inadvisable to eject directly over the area you just bombed - Air Force Manual
Never tell the Platoon Sergeant you have nothing to do.
Never trust a private with a loaded weapon or an officer with a map.
The easy way is always mined.
Teamwork is essential; it gives the enemy someone else to shoot at.
WE SLEEP SAFELY IN OUR BEDS BECAUSE ROUGH MEN STAND READY IN THE
NIGHT TO VISIT VIOLENCE ON THOSE WHO WOULD HARM US - George Orwell