(These are incomplete musings about the major determinants of military power.)
The earliest determinants of martial power were probably the genetic differences between different hominid (sub)species in regard to physical fitness for spear-throwing. The size of social groups (measured in combat-capable men) were likely important as well, and more nomadic groups did probably enjoy the advantage of surprise more often.
An additional, and powerful, advantage appeared with the Bronze Age (in Europe, Northern Africa and continental Asia). Ancient bronze was made of copper and tin, and these ores weren't both mined in a single region.
The bronze was important for war because the first swords, fine axes, effective body armour and helmets and even shields could be made of it, and quite easily so. It's actually likely that swords were mostly wartime items, melted and cast into dishes and other non-martial items in peacetime. Bronze is different from iron in that casting is just fine for almost all purposes and little can be gained by special treatments - unlike iron alloys, many of which depend greatly on heat treatment, for example. So with bronze you could simply recast some civilian stuff into martial stuff and back, rinse and repeat. This was done till almost modern times with "gunmetal" bronze (artillery barrels into church bells and back).
Bronze was important for martial effectiveness, so being able to afford it and having a culture in which the next generation inherits as much bronze as possible (thus no bronze wasted in offerings to lake gods or similar, none is wasted in funerals) were important. To purchase bronze required long-distance trade, so being able to produce tradeable goods such as pelts, ores, golds, amber or simply being able to effectively raid for slaves was an important determinant for martial power.
Bronze lost some importance during the late Roman Empire when iron armour and helmets became more relevant, but it had its comeback with the advent of blackpower artillery, because the gunmetal alloy allowed for more durable, less dangerous (to its users) and lighter artillery barrels (though several times as expensive as iron barrels). This advantage only disappeared during the mid-to-late 19th century because gunmetal proved to be too soft to maintain a rifling in the new rifled cannons.
The martial power of powers such as Babylon or Egypt rested on more than on their ability to trade: Their strengths were agricultural productivity and organization.
A farming family may have enough productivity to feed itself or to feed one or multiple more families as well. The more families it can feed, the more (wo)manpower can be allocated to mining, trading, craftsmanship, arts, construction, organization (aristocracy and bureaucracy) - and martial activities such as training for combat, raiding and waging war. Higher productivity did thus free personnel for other activities than merely avoiding starvation. The higher the agricultural productivity (or the hunting and gathering productivity), the higher was the warmaking potential (ceteris paribus). The ability to conserve foods (by drying, salting, canning et cetera) for campaigns in distant areas was important as well.
You need to exploit this potential, of course. The disappointing progress of African infrastructure and of the Afghan state's (para)military strength show this. I've read many times that Afghanistan cannot sustain a large army of its own because it's too poor. Those authors don't think much, apparently. Afghanistan could easily sustain a two-million men army just as it can sustain two million men (equivalents) doing nothing of critical importance (underemployment). They may not have the organization in place to harness their potential to the end of martial strength, though. But that's not about poverty.
At least one detrimental factor for warmaking capability is noteworthy; dependence on foreign support. The inhabitants of an oasis cannot wage war. Luckily, they usually don't need to. Their dependence on trade with caravans (for salt, if nothing else) makes them incapable of resisting those less dependent potential adversaries. Oasis populations were usually safe because of relationships of mutual dependence and because said inhabitants had such little bargaining power that their only potential enemies did not need force to get what they wanted.
More about contributing factors; it appears that regular exposure to combat forced groups to exploit more of their potential for martial strength; populations on island groups such as Philippines or the Aegean world tended to produce rather rich military histories. Faraway places such as Iceland on the other hand turned even Vikings into relatively peaceful folks. Regular opportunities for raiding and the ability to avoid angered neighbours - such as enjoyed by Asiatic steppe people - contributed a lot to martial prowess, aggressiveness and thus warmaking capability as well.
The economic systems based on money as seen in the Roman Empire(s) and later Renaissance-to-18th century Europe turned the organizational ability to collect and accumulate gold and silver into an important determinant of warmaking potential. Mercenaries and alloys were easily available, and coins became the universal substitute for almost everything needed in war. A country's actual physical warmaking capability and its males' martial prowess became less relevant than the rulers' gold and silver treasure (and ability to collect taxes and tariffs).
Nervi belli pecunia infinita
(Unlimited money is the muscle of war)
Another agricultural factor for warmaking potential is the ability to feed horses. This ability was mostly a given in plains (Asiatic steppes, for example) and also available in flat valleys (such as the ancient Greek Thessaly). It didn't help much to import horses in times of war into a country that's unable to maintain many horses in times of peace: Riding skill takes much practice (the enlightenment age's European cavalry required six months training for its rather modest skills, while effective horse archery on the move takes years of practice). Wealthy countries were nevertheless able to exploit their cavalry potential to the fullest by breeding no or few horses. Adult and trained horses can be imported in exchange for coin and don't require your country to feed them for years till they're useful for riders. The ability to maintain many horses was of great use for thousands of years, but in most of Europe there was a dominance of cavalry only from the migration period (about 4th century, see Battle of Adrianople in 378 AD) to the late medieval age (about 14th century) when a combination of ranged individual weapons and disciplined pike formations surpassed the cavalry as battle-winning forces.
|French ironclad Le Redoutable (1876), uniting|
previously unheard-of quantities of steel
A fleeting substantial factor for military strength was quality steel. Quality steel was rare until modern times, and depended as much on suitable metallurgical practices as on the availability of iron ore of good composition (some iron ores of asteroid origin or from special deposits such as in Noricum were fine). Such quality steel was required for fine swords and for the blades of fine axes. The ability to chop off an enemy's axe handle was definitely of great utility in combat.
Quality steel became important again when European industrial age heavy industries competed for the best armour plating (for warships) and the best armour-penetrating shells. Krupp armour is still famous for its role in the late ironclad era, for example.
Access to nickel, manganese and other additives for quality steels became important factors in the Second World War. Some strategic decisions (such as defending the Kuban bridgehead in 1943 for its Manganese) were driven by this. Canada did apparently produce almost all of the world's nickel production during the Second World War.
Ideology - including nationalism and theism - was probably always and important factor in a groups' ability to harness if not exceed (such as in Paraguay) its warmaking potential. This became once more evident when nationalism enabled Europe to raise military forces of sizes never seen before (except in China or maybe India) since the French revolution (Levée en masse). This did fit all-too-well into an era during which artillery was not yet dominant (and actually about to soon experience a low point for a generation) and cavalry more a reconnaissance and courier force than a battle-winning force. The ability to raise and maintain huge, personnel-intensive armies was the key factor in European warfare from about 1800 till maybe 1950. Ideology as source for motivation (and tolerance for the wastefulness of war) does in part substitute for physical incentives (such as silver).
Finally, during the 1890's heavy and chemical industries became huge factors for a country's warmaking potential. The electrical, automotive and aviation industries as well as crude oil deposits joined them in the 1930's and the semiconductor industry did so in the 1960's (approximately). It was always necessary to produce clothing and other mundane items as well, but this was usually possible in some way or another anyway.
The heavy industries (coal, steel) were usually founded on the availability of either iron ore or black coal deposits (hard factors), while the other industries were more driven by national academic research support and policies (soft factors).
Nowadays Europe appears to derive its warmaking potential mostly from its multinational cooperation, which enables countries to maintain armoured and air forces without the respective industries. We have more than enough personnel potential (albeit we couldn't really compete with India or China in this regard), and fiscal potential is a given despite the rigidities forced on us by the maturity of our societies (with already distributed and only slowly growing national income). Another major contributor is martial prowess, indicated more by institutional heritage of our military forces than anything else.
We cannot count on a superior education and nourishment of our recruits any more.
We cannot count on a superior education and nourishment of our recruits any more.