while I agree that a higher population doesn't equal to a stronger army, but it does give one an advantage assuming the country is in prosperity. There is more work force and more volunteers to support the army. Look at America during WW2, it had the remaining country working for the support of the war. If the country was 1/3rd of its actual size, do you really think America could be successful? Probably at the very least less successful. On the other hand, if a country is weaker or ill equiped, then a higher population can be a headache. The example includes the Soviet during WW1 when they were lacked weapons to use, and of course they had high casualties.
In the case of the current period for the Tang from Zhenguan to early Tianbao, the more the merrier for the army.

Ok, then why Mongols conquered half on then known world with a tiny population and almost non-existing economy? The key is tactics, military organization and military know-how. And both Mongols and Byzantines had these assets.

There was no Soviets during the ww1, so elaborate what are you talking about.

True, but all I was merely saying was that population with a prosperous country=more military might. Why is this not true? While, I agree that tactics are important, but having a high population plus good country should make the military might even stronger. If the Mongols only had 30,000 army and tried to conquer the whole world, then I doubt it would stretch all the way into Hungary because no matter how fierce the armies are, you still lose a proportion of you army due to casualties in each battle.
Your second sentence is not revelant to the population question of whether population equals more military might. I agree that your keys are factors, but not the overall factor. As mentioned by warhead before, resources is also critical. I don't care how powerful the Germans were during WW2, but they either way lost in the end to U.S due to a lack of resources. In the end, General Rommel was lacking fuel to use his signature tanks and the # of tanks+weapons+soldiers available were significantly lower. On the other hand, though U.S may possibly have suffered some serious casualties against the German firepower, the U.S was like an wave that just couldn't stop getting you. You can shoot one plane down, but at the same time 2 more were just manufactured over at the U.S. Sooner or later, the Germans would get over matched. The same problem arose for the British, they lacked the weapons. So what did they do? They bought weapons from the U.S and eventually after France fell, FDR made the lend lease Act and sent tons of destroyers to the British. If the British didn't have the resources that the U.S was giving it, do you really think the British could have held on against the Germans? Dubious.
My mistake, I meant Russia. Soviet didn't exist, but Russia did. Later Lenin did withdraw the troops after the Communist take-over; however, before that, they had a similar problem that lots of country had- the resources. Before the peace, Russia were senting in troops with inferior weapons and to make matters worse, some didn't even have the proper weapons because they were lacking them.

It was actually 30,00 Tang forces and around 40,000 Ghazis with an unknown number of allies.



No, the key here is that the Mongols have a different system of organization; they are nomads, while the Byzantine empire was not. The comparison here is hence invalid.
You need to understand how nomadic empires function and how they are fundamentally different from sedentary empires. Nomadic empires rely on agricultural states for sufficiency through trade or pillage. They are not self sufficient. Yet because they have no need of farmers to produce surplus to feed their army, a much larger portion of their total population could join the military. While around 1/50 individuals in an sedentary empire were enlisted as soldiers, as much as 1/4 of the entire nomadic population could participate in the army.
The second advantage of these nomadic armies was their mobility. This can be attributed to two factors; the abundance of horse and their wandering lifestyle. According to estimate, Mongolia has over 3 million horses, which constitute around half of the entire world's horse supply, every soldier has 5 mounts which enables them to cover long distances without the need to worry about remounts. Secondly, the lack of cities in nomadic territories allows them to be elusive and maintain a strategic(not tactical) advantage. Whenever the nomadic armies actually faced Chinese armies on the battlefield, victory could easily go to either side. But on the big picture, nomads could easily gather a large force and concentrate their attack on isolated Chinese strongholds, and due to their mobility, they can easily retreat when sedentary reserve forces come to the rescue. Hence in most battles, nomadic forces outnumber Chinese forces in that specific region. Chinese offensives against the nomads were made difficult due to logistic problems, Mongolia is a huge area with no cities to loot. Long supply lines were needed to conduct a campaign. The nomads, with their superior mobility and sparse population could easily avoid battle with the sedentary armies until the latter were exhausted of their supplies and were forced to retreat. Therefore, the initiative lies with the nomads.

The Byzantines has no such advantages. They are sedentary, with cities to be captured, they are more predicatable. And population plays a big role here. Their horse supply only numbered around 200,000 during Justinian's time. And significantly reduced by the 7th century. The Tang on the other hand had over 700,000 imperial stallions, with an even more impressive private horse breeding industry.

There were a few major factors in an objective assessment of the strength of ancient states. Population was one, terrain was another, crucial military resources such as iron and horse was important as well, yet administration was perhaps the most important. Smaller states were able to overwhelm bigger ones due to their greater centralization, their ability to effectively execute political and military actions, and the ability of the state to conscript its population. This was the major reason why states such as France lost in the initial phase of the 100 years war with England, they did not have the advanced bureucracy to utilize the advantage of their greater population and conscript them for war.

Yet in all these areas(except terrain), the Tang state was unrivaled.
Modern bureaucracies requires 6 fundamental aggregates; a board for foreign policy, aboard for economic policy, a board for administration matters, a board for civil matters, a board for military matters, and a board for legal matters.
The Sui-Tang empire's 6 Shang Shu already satisfy the requirements of a modern bureucracy; the board of rites dictate foreign policy, the board of punishment dictate law, the board of registration deal with registration and agricultural matters, the board of Shi deals with promotion of magistrates, the board of military deals with the military, and the board of work deals with construction projects.
The Tang state was more centralized than any in the world, its equal field system allowed it to keep a fairly accurate census of its population, which implies that it could conscript them readily for war. The Tang horse breeding industry was the most developed on earth, and the same can be said about its iron industry, which had the blast furnace technique that produced as much as 20,000 tons of pig iron. In none of these areas, does the Byzantine come even close.
And as I already asserted, Koguryo's strength in these areas makes it a power comparable to the Byzantine.




The Opium War was a different story, we are entering the age of industrialization where weapons of advanced technology became the decisive factor. Prior to the industrial revolution, weapon disparity were miniscule over most of Eurasia and were not decisive in determining victory. Yet with the introduction of steam boats and the later breech loaders, technological disparity widened to a degree that was unprecedented.

The Byzantine military was only the caliph's major enemy in the 7th century, by the 8th century, enemies such as the Khazars and Turgis were already overtaking their importance.

I found several Japanese sketch of Tang Maps. Its even larger than the ones China drew. Apparently, they've included all of Tang's vassals. Bombastic as it might be, there are a lot of details which even the Chinese maps lacked. The only other empire with such a size I've came across was the Mongol Empire.

http://tt.mop.com/club/read_2200136.html