In wars, would an enemy be unstoppable if it forces a population in every captured territory to join its army?

Alexander the Great, and the Roman empire used this method- when they invaded a land, they offered a choice to the males- either join, or die. As you can see, the choice is not very broad, so their numbers swelled.

Also, save for a few places, Alexaderian/ Roman armies were welcomed wherever they went (after they had taken a break from wholesale murder and rape, that is). So it was easy for them to swell their ranks.

An interesting piece is the Roman Empire’s army- contrary to what’s shown in Hollywood, Roman soldiers were not all Chiseled, Nordic blondes. Roman armies consisted of soldiers from Europe, Middle Eastern Arabs, Central Asians, and Black Africans.

Roman Army, c. 540 B.C.

Do you know there even was an African Roman emperor- Caracalla!

The Black Emperors of Rome: Roman Emperor Caracalla (Kara-kalla)

Coming back to your question, my answer is NO.

You see, in the modern battlefield, numbers don’t mean shit. The number of times in the history of modern warfare a smaller, more disciplined army has defeated an enemy many times more stronger, is, well, Countless. And forget the first or second world wars- look back, WAY back.

Raising a smaller but disciplined. regularly paid, well armed and well trained army was exactly how the British empire invaded the known world 350–400 years ago. Defeating an army 2–10 TIMES larger was…routine for them.

Battle of Ferozeshah – Wikipedia

Or how, in 1990, the US military curb- stomped the fourth largest standing army in the world.

So, in modern warfare, numbers do not matter. The ‘let the vanquished join’ strategy was useful for the Romans because they needed sheer numbers, to not just invade, but also to protect what had already been invaded.

Now-a-days, armies don’t walk together like a wall. They are spread out over the battlefield. Some are hidden, some ready for flanking. The different arms move independently (but coordinated) from each other. Such a strategy is possible only because the army as a whole should be disciplined and perfectly willing, and able, to carry out orders, how and when they have been told to.

An army of brainwashed locals is-

  1. Difficult to raise- it will take a LOT of time to brainwash the locals.
  2. Extent of brainwashing in two people is never the same. So you will have a totally brainwashed zombie depending on a potential mutineer.
  3. Brainwashed troops can never think or judge for themselves. The greatest strength of the US Army is that soldiers and NCOs are repeatedly told to think and judge situations for themselves. Grunts are not supposed to be dumb oafs.
  4. Such troops will take high casualties, and thus will burden your field medical infrastructure.
  5. More soldiers mean more mouths to feed and more guns demanding more bullets, burdening your logistical chain.
  6. But they definitely do not mean more EFFECTIVE soldiers. ISIS was so scary and terrifying on TV, with volunteers joining from all over the world, but got its ass handed to itself when it went u against US/Russian Air power and the renewed Iraqi Army.
  7. A LOT of potential mutineers and deserters. Your army will deplete faster than you can build it.
  8. All locals might not be healthy, so you have to conduct medical checks on EACH- I know you don’t care about their well- being, but what if one of them is carrying an infectious disease? What if your entire army catches the Flu? Or Chicken Pox? Small Pox? Cholera? Leprosy? Even AIDS?
  9. You cannot rely on it to function or think independently, you cannot leave this force alone for flanking the enemy, because you doubt it so much, that you have to maintain constant supervision.

All in all, a terrible headache, a huge sink of resources, that does not result in a content, disciplined, well armed force. So, not a good idea. A lot of ado for nothing.

Edit: A better idea than brainwashing locals, is to invite the locals to enlist for you. That way you attract the few who already support you, so there is no ideological or security problem. Also, you attract few, so you can pay them well, and arm and train them well. And of course, it is a huge propaganda boost for your side.

If not for ideology, They will join even for the regular pay and food and clothes- which are a luxury in any recently invaded land.

Frenchmen fighting for the Germans for the above reasons:

No.

In fact, it would guarantee defeat of that enemy force when they faced a competent opposition. Success in combat is not primarily about having the most bodies. You need to have people who are motivated to fight, who believe in the mission, who think those on either side of them will have their back.

Do you know why there are so many Pennsylvania-Dutch in Pennsylvania? Most of them are descended from Hessians. Do you know who Hessians are? The British didn’t have enough troops so they hired them from Germany. These troops were forced to come to America to fight. Most of them did not see it as their fight, they didn’t identify much with the British or the leadership provided by the British. They deserted in droves and eventually settled in that part of the colonies.

The Germans had a similar experience in WW2. They sought to create units from POWs, and residents of any nation they conquered. Some fought well—but mostly b/c (like Vlasov’s White Russians) they hated who they were fighting more than they hated the side who had conquered them. Most of these units easily surrendered and showed little fight.

Motivation and professionalism is critical for successful military, especially combat infantry. Demanding that people join you and fight for you by pointing a gun at them is guaranteed to produce an inept military with no trust, no motivation, no espirit de corp, and no commitment to the fight.