Between attrition (lost in battle), needing to leave troops behind to control conquered territory, and longer supply lines, shouldn’t the armies have gotten significantly thinner and weaker.
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It depends on the era that you’re considering. Morale/confidence is still a significant factor in warfare but was massive for melee combat, so an army that won several battles was likely to be more confident. Being seen as strong/victorious also helped recruit local allies and mercenaries.
Detaching troops to garrison captured territory was sometimes a problem, but many armies simply didn’t do it. Keeping force concentrated and being able to retake the territory if needed was often easier than dividing the force with enough strength for each portion to be effective.
Ancient armies frequently lived off foraged and captured supplies throughout a campaign, so supply lines were less critical than in recent wars.
No context. Is there a specific situation that you’re asking about.
In WW2, the German army conquered lots of Russia but ended up losing because they ran out of fighting strength. So this is at least one counterexample.
I would imagine there were a lot of “fight for us or we’ll kill you and your entire bloodline” moments. Kind of like that scene in ‘300’ where Xerxes offers Leonidas to be warlord of Greece if he surrendered. If any of these territories did surrender and choose to fight for their new leader, then you basically inherited a new army, and would probably only have to leave a few behind to fight off any dissidents.
What sort of conquest are we talking here?
Let’s split wars into two broad types: Wars of two large countries, and wars of a large country invading a series of small ones.
In the first case, conquering land is good because you probably defeated your opponent to conquer it. Sure, you lost perhaps 10% of your forces, but the enemy lost like 20% of theirs. Then at the next battle, your enemy is less trained and weaker, so you lose less and they lose more. By the time you’ve conquered a lot of land, your equal opponent has fallen far, far behind. Maybe your opponent can pull in reinforcements or conscripts, maybe those factors you mentioned kick in, but maybe they don’t.
In the second case, having more land means more industry and more population. You can start fielding larger, better-equipped armies, who conquer more land, who give you more industry… This isn’t foolproof though. The Romans, for instance, eventually failed to conquer certain lands such as modern Germany and Scotland.
Do you have any examples of this you’re particularly interested in?
There are a range of reasons why it can happen. For example, they may gain experience, or be augmented by forces or resources from conquered areas. Often in history “changing sides” isn’t that much of an issue for soldiers or their leaders (indeed, it’s probably a good strategy to invade the most sympathetic areas first – they might even welcome you). On the flipside, their opponents are likely to suffer the opposite effects, becoming demoralised, disorganised, losing resources, etc..
However there are plenty of factors that work in the opposite direction so I don’t believe there’s a general rule here.
It may be a kind of selection bias you’re seeing, because conquests tend to stop once armies start losing battles. Actually gauging the strength of an invading army is tricky – are they winning battles against stronger opponents? Are they winning battles more easily against opponents of the same strength?
Without context, your question is too broad (there are many examples from the Ancient world, Ancient Rome/Greece, Medieval world, Renaissance, etc. that prove and disprove your assumption that armies ‘get stronger’ as the conquer)
Are you thinking about modern armies (ie post 1700AD onwards?) or medieval ones (600AD-1500AD) or something else?
You are also assuming that supply lines are a necessity (they are in modern wars, but ancient warfare didn’t require such a focus on logistics, they didn’t need a constant supply of gasoline for example) and that armies are not reinforced as they conquer, which isn’t the case (after conquering a foothold, many armies bring reinforcements and make up for any losses over time)
The basic premise of the question is a bit flawed because that’s not always the case. But on a basic level the main factor is that if they’re gaining more land, it usually means that they’ve caused very significant losses to the defending side. If the defenders have lost troops, equipment, strategic positions and/or important locations such as manufacturing hubs or population centers from which they can no longer replenish their ranks, it can reach a point where the defenders have simply been pushed past the point beyond which they can recover, and as such this can snowball into them no longer being able to mount a good defense and the attackers can capitalise on that and rapidly expand their controlled territory. There’s also a morale aspect because an army that’s seeing massive advances and victories has a lot more confidence than defenders who lose their nerve as they see this seemingly unstoppable force coming towards them.
However this is far from the norm. Throughout history many armies have made the mistake of overextending themselves beyond their logistical capabilities. Napoleon Bonaparte had famously said “an army marches on its stomach” which can mean many things. On a surface level it means that an army can only be effective and have high morale when they’re eating well, but on a broader level it also speaks about the basic logistical capabilities of an army. If you can’t reliably supply your troops with food and equipment past a certain point, then any gains you make past that you cannot actually hold. This is a very dangerous pitfall for an army to find themselves in. As conquest expands outwards you need more and more combat troops, and for every single combat soldier in the front lines you need a lot more support troops. This is called the “tooth to tail ratio”, and the number can be anything from as low as 1:2 to 7:10 or even more. This means that the more you expand your support and logistical needs expand exponentially both in man power and equipment needs.
So basically unless an attacking force can deal a swift and decisive blow to the defenders, from which they cannot recover, expansion is actually a bad thing because it stretches an army’s capabilities thin.
When you have a new army, lots of those people are fresh troops with little training.
After a few battles, your army has less people, but not much less effective fighting force. You’ve just shed all the troops that weren’t going to be much use. The ones left are the best at fighting.
So now you’re much more mobile, require much less supplies, and you’re almost as good at fighting as you were before. On top of that, your morale is high and your intimidation is high.
The question seems flawed. But it can happen.
For example if we look at Alexanders army his commanders became more experienced and confident, his troops had a morale boost since they seemed to go from victory to victory and he picked up lots of mercenaries (like persian archers) on his way.
But then Alexanders campaign was studied for 2000 years due to how well it was planned in terms of logistics (taking a rather convoluted path so that it never overextended its logistical train).
Before modern times times there was also the limitation that you can’t let an army sit in your own territory for long or grow too big. They’ll eat up all your stockpiles of food! Then when the army is in hostile territory (and having outriders that loot and pillage all of **their** resources and bringing it to your troops) you suddenly have a lessening of pressure on your own resources, so as more food is stockpiled from your own territories you’re free to gather and train up replacement troops and send them in reinforcement columns to support the main army. And that frontline army will have now fought battles and skirmishes and gained battlefield experience.
In many wars victorious armies could attract support from neutrals who wanted to enjoy the benefits of the winning side. There could also be rebellions against the losing side, and the rebels would help the invaders in return for promises of independence or privileged positions once the old regime was defeated.
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