Gfrktyl

Women and children start fighting.

Imagine something like Japan at the end of WWII. Up to this point the army was fighting soldiers with weapons. As they enter the cities, they meet a different kind of opposition. Young kids with guns. Women charging at the army with hand-made spears. The remainder of the armed forces using body shields of people too young to fight and POWs. No one will surrender. To win they have to kill 100% of any city's population. This will demoralize any army.

The losing side becomes desperate, and switches from using formal military to guerrilla tactics. They brainwash their population to perform suicide attacks. At the end of WW2, Japanese civilians were known for committing mass suicides as US troops were about to occupy their town. But in your story, government provides civilians with both conventional and banned weapons for use on invaders.

• Biological weapons, e.g. civilians infecting themselves, and then simply spitting on enemy soldiers. Or bringing infected ticks or lice into their camp.




• Bombs implanted inside the body. "Pregnant" woman is a popular method.




• Kids throwing grenades from rooftops.

And so on.

Urban or guerilla warfare will do the trick quite nicely. Having been easily cut down in conventional battles, the losing side turns to asymmetric warfare to try to hold off the invaders. It's easy to see what makes this a desperation tactic: they're practically daring the attackers to destroy their cities, oppress (or displace, or simply wipe out) their civilians, and generally grind the nation into dust just to get them to stop. Even if the defenders "win", it could take generations for their country to fully recover from the damage the war causes.

The hard part is justifying why the invaders aren't expecting this move, and why their response is so bad as to become a scandal. (There's no scandal in the counter-insurgent phase of an invasion being long, drawn-out, and painful to all sides: that's just how it is. But there's definitely a scandal in gross mismanagement.) Perhaps there was a generally-accepted idea that the defending nation didn't have it in them, politically or socially, to continue fighting at such a late stage: the attackers thought the government would give in much earlier, but it didn't for whatever reason (different leadership? a coup?) and now the resistance is much better organized, equipped, and motivated than anticipated.

Edit per comments

You indicate in your question that the invading military and intelligence apparatus is thorough, methodical, and very strategically-minded. This makes it unlikely that they missed preparations for a national-scale insurgency following a conventional defeat. Such a thing takes preparation and foresight, not to mention lots of people. It wouldn't remain a secret. So in order to be taken by surprise, either the intelligence community misinterpreted this information, or the military or political leadership willfully denied it.

The former could be due to some kind of bias or unspoken assumption. If the attackers have spent years portraying the defenders as weak or cowardly in their propaganda, analysts might subconsciously believe it even if there's no basis in fact. Conversely, they might assume that the defenders will follow their own (that is, the attackers') beliefs and values. One theory about the ultimately disastrous Japanese grand strategy in WW2 was that they misinterpreted US strategy in WW1 as being primarily motivated by cowardice (as opposed to a combination of greed and apathy). The projections of their analysts therefore didn't match reality.

The latter problem is mostly a matter of politicians and military leaders wanting the war whether it's practical or not. Perhaps the political climate is such that reasonable voices are being crowded out, or subordinate officers are rewarded for being "generous" in their estimation of their superiors' plans and their odds of success. The leadership might be focused on the easy initial stage rather than the difficult later stages. There might be a (perceived or actual) need to downplay the risks to buoy the morale of the troops or the civilian population. Whatever the case, although the leadership has or should have all the information they need, they don't reach the correct conclusion.

Quite a few examples can be found in WW2

1) Battle of the Bulge - In a last ditch effort at the end of the European theatre, the Germans threw everything they had in a last ditch defensive/offensive. So much so it took the allies by surprise forcing them to concede ground. The Germans marshalled the entire male populace from boys to the elderly in the operation. Naturally the body count was quite extensive on both sides.

2) The Japanese - So this was more the entire Pacific theatre but it certainly got worse the closer they got to the mainland. Basically, the Japanese cultural sense of honor was so intense that their fighting spirit completely shocked allied commanders. As they got closer to the mainland it quickly became honor and to protect the motherland. They would literally fight full force to the last man no quarter given or expected.

--After WW2---

3) Afghanistan - The western forces literally pancaked the Afghani military forces in like weeks. Yet the war raged on for years due to a modern form of guerilla warfare. The onset of modern media has made it so that public targets cannot be attacked without incurring huge sums of public sentiment. Thus this modern version of guerilla warfare uses civilians to camouflage attacks. This tactic only works because of the media because in the past such attempts were met with methodically cold acts of what would be considered today 'war crimes'.

There's lots of tactical arguments posited so far. I'd like to offer something slightly different.

There's nothing quite as traumatic as one approaches final decisive victory than to realize you've been fighting on the wrong side all along.

Perhaps the winners start to realize their mistake, but due to the inertia of nation-level combat, they are obliged to continue grinding away at the losing side, knowing exactly how history will write of them.

History may be written by the victors, but that doesn't mean they have to like what they have to say.

The Fifth Column

What if Hanoi Jane wasn't just an actress and activist, but also the face of a hidden army of militants lurking at home? When OPFOR's victory was still considered a possibility, this network of enemy agents tried to keep themselves largely hidden from public attention, sticking to covert operations like intelligence gathering and minor sabotage.

Now that OPFOR's defeat is written on the wall, the fifth column decides to go out with a bang. They quickly arm themselves and openly strike high profile targets both military and civilian in the rear area. The supposedly safe homeland isn't so safe any more with the sudden emergence of a home front.

Given that most of BLUFOR's troops would be deployed in foreign soil, the army will be ill positioned to stop this uprising. What would ensue is a campaign of terror waged against the winner's civilian population as the army rushes troops home to deal with this new problem. Inhumane tactics like those used by the Taliban and the WW2 Japanese can now be displayed directly to the civilians without the need for the media to bring these images home. This would create the mental trauma you need due to troops having to turn their guns on their own people, and the political scandal after the war ends.

At the start of war everything was under iron grip and went by great plan. So Army went from A to B and had good moral, logistics, discipline even part of enemy population greeted them with flowers in hand. But at the end of war that great plan went wrong. Flood/enemy general or they by mistake damaged huge dam, as a result soldiers and civilians die in huge numbers. All that is used by enemy to try and change the tide of war. They spread propaganda and start desperate atttack on cut off army. At the same time bursts festered problem of rear army: while generals had hard control on battlefront and routes of logistics, their reserve troops and mercenaries who were placed behind used lack off supervision to get rich sometimes even rape and kill. While it was rare and mostly covered, people knew. After the flood it spiked and under propaganda, it boiled off and started guerrilla warfare.
So invading army had to deal with cut off army, flood, huge dip in moral, lack of land based logistics and loss of good part of supplies and over that desperate enemy counter attack and guerrilla warfare in the rear.

The defenders have weapons of mass destruction such as nuclear or biological weapons.

So long as they thought there was a chance to win they didn't use them, but once all hope was lost they deployed them in one last ditch effort not caring for the consequences.

The winning side has had good and reliable evidence that the military campaign was against, mindless clones and programmed machines. The civilians believe this too, the evidence appears legitimate to everyone (almost) in power, everyone on the ground, and every civilian.

The war effort progresses, side A winning, not an out right victory but no slog either. Everyone is wishing to put this all behind them.

While the war effort has been underway, many civilians have been recruited to work in factories, bio-labs, etc... manufacturing war-machines, and researching the means to heal wounded soldiers, such as cloned-limb replacement.

Suddenly the forward march of the army slows, the machines have gotten tougher, the clones smarter. They start to loose some ground. Nothing too serious but definitely alarming. The military reorganizes and plans an ultimate assult on those last strong holds.

The factories work in overdrive, the labs grow tissue like mad. Everyone has pulled together to make this offensive possible. The machines are second to none, the tissues are super-enhanced.

D-Day.

Guns pound the ground, missiles launch, tanks grind over walls, nothing but blood, death, and cruel land-acquisition. The battle is the worst yet, no one makes it out without some wound - but the day at last is won.

But there are questions now. How did the loosing side, so clearly under-resourced, so clearly out matched, suddenly field such a an almost overwhelming display of power. If it had of been unleashed even a day earlier they wouldn't have won.

In move the investigators, they pour over the technology they just defeated. They fly in experts from the factories, from the labs to analyse and assess.

A technician realises with horror what the twisted wreck of iron and flesh in front of him was. He had designed it himself.

News is reported back to the home land. How did the finest military equipment, built by the finest engineers, get used against their own forces...

The weaker side, anticipating total defeat, used weapons that were illegal or thought too horrible to use. This can include nuclear weapons, poison gas, and weaponized anthrax.

The stronger side has kept the fighting more or less in the countryside for as long as it can, but concludes it has to take cities to finish the war. They find the cities very well prepared for defense, with lots of extra defenders. They're not very good and only have personal arms, but they can still be quite effective defending in a city, and their morale is high because if they lose their country is conquered. There are also lots of booby traps prepared, as the conquest has been predictable for some time now. The invading commanders had been aware of this possibility, but had been putting it off as long as possible in hope of getting a surrender.

The weaker side managed to talk another country into an alliance late in the war, and the third country gets into position to fight shortly before the end, operating with considerable stealth to surprise the invaders.

Maybe as an alternative: what if it isnt the enemy that causes the trauma?

Its basically mandatory since early history to always make the enemy the bad guys. Even the Nazi's made it seem that their atrocities were justified to the people executing those orders and the lesser atrocities to the entire human populace. The Americans in the civil war fought openly for slavery and in Vietnam the coming of independent war newsreports revealed the atrocities both sides commited in the war.

In your scenario the tired, jaded soldiers might celebrate their victory through looting, rape and murdering of prisoners. They might "pre-emptively" find and kill people who they deem a risk of becoming guerilla's or they might simply look for extremists and legitimate threats and execute them without due process, scarring the countries idea's of why they fought there.