"Old soldiers never die. Young ones do."
This was a response to the famous line from Douglas MacArthur declaring that "Old soldiers never die. They just fade away." The idea is that old "soldiers" (if they can even be considered soldiers) aren't the ones that die on the battlefield, because they don't do the actual fighting. It is the young soldiers on the front lines that do the fighting and the dying.
"War hath no fury like a non-combatant"
This was a quote from author and journalist Charles Edward Montague. Often society is divided into politicians and "opinion makers" trying their hardest to push the country into war and the soldiers who actually have to do the fighting (and are often prevented from leaving the military when war is on the verge of being declared). The soldiers are the ones ordered to risk their lives, while the politicans are the ones who put the soldiers' lives in danger from the comfort of their desks.
To the soldiers on the front lines, the rear echelon mother fucker who is a commanding officer follows the same "illustrious" tradition. While they hide far away from the enemy, they make decisions that have a direct impact on the lives of the soldiers - some of these decisions may include sacrificing units in the name of saving the overall campaign. Rarely do the units sacrificed include the REMFs making the decisions (unless there are even more rear echelon higher-ups than these, like the Commander in Chief).
The following was a quote at Wikipedia at one point, but has since been deleted:
REMF : rear-echelon motherfucker. Widely used in the Vietnam War, it was intended to refer to anyone with non-combat jobs, including nurses and doctors at China Beach or supply clerks; however, the real acrimony of the phrase was directed to the high-ranking officers at Military Assistance Command and The Pentagon, who sent soldiers into combat at no risk to themselves: General William Westmoreland, Ambassador Maxwell Taylor, Secretary of Defense Robert McNamara.