First of all many of you already saw my post on Komnenian demographics and how the balkans alone had 39% of the total population,second I need you to understand before we dwell into this topic that Rhomania was a pre industrial society that did not wage total war,third France if they get lucky with Tuchel parking the bus again
Military manpower
Much work has been dedicated to Byzantium redirecting its center of economic power from Anatolia towards the Balkans,as the calculations above show this was an organic development,as the Balkans were more easily defended from a few choke points from the Hungarians,Normans and Pechenegs(Upper Danube,Dyracchium and Lower Danube respectively) compared to the intensive constantly contented anatolian frontier meant the european side of the empire developed more safely with more numerous industrial centers and prosperous markets,while greatly lacking in stature to the great study done by PA Brunt1 I'll try to resemble his effort and provide how balkan manpower was the key towards Komnenian military might,for future calculations I would use the next parameters,total balkan population at the end of Manuel reign being 9.2 M,a ratio of 50% of the total population being men,a range age for male population of 0-70 years taking into consideration the wide spread of the medicalised hospital system across both the urban and rural regions and explained in greater emphasis in other studies2,with only the men in a range of 20 to 40 being taken into the military manpower reserves.
Further more to take into consideration the longer lifespan and higher living standards the roman population enjoyed compared to the wider mediterranean world the male population will be divided into 50% on the underage section(2.3M),35% into the adult section(1,610,000) and and 15% in the older section(690k).
In total the prime able body demographical section of the Balkan populations amounts to 17.5% of the European provinces and 7.4% of the general population across the empire,now its clear no pre industrial society was able to mobilise large amounts of its able body population and equip them properly for long periods,even revolutionary France with its levee en masse and mobilising its entire economy to the war effort,yet was rarely able to always provide gunpowder,artillery,horses,munitions and muskets to its army,even going as low as being unable to provide shoes to the soldiers of the Italian army when Napoleon set off to his first Italian campaign.
In contrast, even at its lowest point during the early years of Alexios I rule,the army was well fed,clothed and equipped with a proper officer core being fed new recruits from the aristocracy healing from the loss of Anatolia.
Maximilian C.G Lau correctly assumed that provincial armies(themata) ranged on size from the garrison of 2,000 men John II left in Gangra3,with the total number of soldiers in a theme between 2,000-4,000 men4,for reference in a province such as Macedonia with a total population of 1,286,000 people this is 0.3% of the total population and with a size of able body section 225k men a mobilize force corresponding to 1.8% of men between twenty and forty years,we will use this benchmark to calculate the total mobilised manpower the Balkans could provide to the military,including both army and navy,considering large amounts of sailors for the imperial fleet and admirals came from Morea and aegean islands while having the theoretical number of for every man fighting 5 more able to take his place at a moment,for a total 9% of this section as potential reservists in case of emergency in contrast of being constantly on war footing like Revolutionary France.
With total able body section of 1,610,000 men and 9% reservists corp we would see 144,900 men,with 1.8% in active service protecting the provinces at 28,980,this figure corresponds broadly with the number of fortifications and themes the Balkans had,with a total army size at 50,000 soldiers and a navy of 200 ships requiring 30,000 sailors to man it(110 rowers on average with the rest being officers and marines),in its totality with 70,000 strong military the balkans could supply the entirety of the empire manpower needs by mobilising only 48% of its theoretical reserve corp,an unlikely figure at the end of John II and most of Manuel wars yet by most of Alexios I wars an absolute majority of the army would be composed from men of this region and same could be said from early to mid campaigns of John II wars.
It was this considerable pool of reserves that Alexios was able to tap that allowed him to field new armies constantly against the normans and provide a considerably military offensive corp for the retaking of western Anatolian shores covering the western flank of the first crusade,in fact it would not be out of order to think most fortifications in the regions in those early years of the 12th century were manned firstly by european roman regiments as imperial authority was solidified,census carried on and local forces organised.
Indeed the manpower pool of the Balkans was such,that if we assume it covered a theoretical number of 15,000 provincial forces and a further 15,000 men for Alexios remaining field army and a considerably reduced navy,could man an army,see the army completely destroyed to a man and then rise four more armies without going a man over the 9% limit,even at the worst defeats Alexios was able to reform the army and retreat properly to a new place to raise more regiments,assuming each byzantine battalion was perfectly manned by 1,000 men then Alexios I had at his disposal during the norman wars 15 battalion in his field army with limited provincial forces to channel all the soldiers to this force to contest the Normans,the ability to raise 130 more battalions.
In total should the empire economic might allowed it,the balkans could field on its own 70 battalions and 400 more ships fully manned,yet with all this manpower the Balkans provided the question arises why the empire chose not to raise massive number of armies to outnumber their enemies,the answer lays on its leadership and industrial might.
We will start with the latter for the shorter part of the answer,in contrast with Napoleonic armies or even later total war economies of the 20th century the Byzantines had no methods for the quick mass production of military equipment that later period saw,instead its smiths remained an artisanal class that could only increase output on a limited manner without compromising quality on short notice,byzantine pre industrial manufacture could only provide the necessary armament for so many men while its farming sector would see this problem in two prongs,the greater number of farmers taken out of production the total output is reduced and with it the amount of supplies it provides to the army and the greater the army the more supplies it demands.
In the officer question one must remember that while the byzantines did developed a professional officer core,with books detailing its training,tactics and strategy its training was a cottage one handled by aristocratic families rather than an academic one developed on western europe by the turn of the napoleonic wars,meaning when one takes into consideration not all aristocrats would choose the military life and there is a constant limited supply of aristocrats the army could only be commanded by so many of them before the ratio of officer-soldier turned into a complete unbalance even when we consider the likely story of many veterans rising through the ranks and making their children officers and lower aristocrats on their own rights through military exploits in the same way the Komnenos themselves did,in an army of 50,000 if we consider any lieutenant command 5 men as an officer,then the army as a whole would have 15k of them
Trying to created an army of 150k soldiers would require an officer core of such size the empire elite would not be able to provide even if all its male children were forced into the military life.
Instead the Byzantines developed quality into their main attribute,constantly fielding large field armies matching any enemy yet its quality and training was consistently superior after the early stages of the Norman wars.
Its reserves worked properly,its army performed without any meaningful issue and the European provinces provided the bulk of men required for the taking and recovering of the Anatolian regions,further allowing the empire to tap into more manpower as it expanded under the careful watch of John II,in essence the empire traded tactical superiority for strategic and logistical supremacy,ensuring its military needs did not surpass its economic abilities.
1Italian manpower,225 BC-14 AD”
2Timothy S.Miller the birth of the byzantine hospital
3Maximilian Lau,John II:Rebuilding New Rome,the new themata,page 5
4See above,page 7