Ever wonder why one army wins and an other army loses? Sometimes it's not just who makes more tactical errors, or even strategic ones. The biggest error any general can make in battle is ideological in nature.
Technical, logistics, tactical or strategy errors committed due to political homeland pressure or by a nagging dictator.
Please note: the persona of Dictator could be represented by the demos, as was during the Viet Nam war and which lost the war for the Americans. A dictator is not always an Idi Amin or Joseph Stalin or an Adolph Hitler. Furthermore, a dictator may be right and just, not stupid and cruel like the American people were just and right when they pressured the White House to end the war.
Underestimating enemy power and overestimating your own.
Classic example was the 2-million-head strong well equipped and well trained Persian army of Xerxes, who made the fatal error of stupidly thinking they could take on a handful of Spartan warriors.
Your allies all of a sudden side with your enemy, in unison.
Happened halfway in Iraq. Other than the Brits, every other nation found it immoral to attack a country for no real reason, and therefore refused to side with the US. Bush II was pissed off, but the US nation failed to see the message that was given to them loudly and clearly by the entire world: Their dictator was a cruel, evil idiot (Bush II, that is.)
Getting surprized by the enemy. In an ambush, or by a party thrown for your birthday.
Taking the missus or the misses to the scene. They may get sick, you know, upset, due to the amount of blood the good looking young soldiers lose in the ladies' own plain view.
Plus, your own troops will be distracted from the serious job of focussing to gash in the heads of enemy troops.
Getting drunk or high before the battle winner is announced by the judges. It's never over 'till it's over.
Having got into an argument with their sons or daughters to clean up their rooms before leaving for the battle that morning. Domestic disharmony of the leadership destroyed many a mighty nation.
Underestimating and downplaying the fierce fighting capabilities of women. Women are weaker physically, but bend less under enemy torture, because they are more resolute in their faith and commitments
They also keep their holding cells in cleaner condition than men POWs, thus increasing their chances of survival, simply via better hygiene.
Starting wars that are doomed to failure for any number of reasons. Invading stronger countries, wars in far foreign lands, and invasion of countries for no or very wrong reasons.
Russia was invaded three times in a very major way in the twentieth century when Russia / the Soviet Union was very weak. It shrugged, and the enemy went bouncing off. Afghanistan, Iraq, were invaded, and they don't fight back. At least not in an organized, and notably detectable way. Impossible enemy to defeat. The Viet Kong were fiercer and more determined fighters than the Amys whose heart was not into killing, but wishing to be back with their sweethearts, their parents, getting on with their own lives that was robbed of them, or else into LSD. Invasion of America was the only war that was won from a far place. In the fifiteen and sixteen hundreds, not now, dummy.
Biggest mistake ever, but often committed: Subscribing to the wrong religion before the battle, and praying to the losing god for His help to lead the troops to victory.
I condemn war unless it is fought for the survival of a nation. I don't condone only defensive wars; an aggressive war is just as just if it is the only way to ensure the survival of a nation. That said, I am a pacifist, never fantasized ever about killing people (my gym teacher and Mr. Hubber, the Calculus teacher being sole exceptions), and I don't think I am capable of killing or physically hurting anyone else. Not even killing myself -- I tried, many times, and failed at it, miserably.