STRATEGY
The Turian Hierarchy
Turian military strategy in the first half of the 2100s CE can be described in three words; cautious, methodical and relentless.
Its caution was entirely the joint product of the Krogan Rebellions and turian separatism, a trait that descended from the very highest levels down to the company level. This in turn led to the methodical approach applied to every aspect of the turian military, with tactical approaches to any military problem having been boiled down and practised on. Turian military manuals remain famously large tomes to this day, and have only expanded in the face of humanity's arrival and the threat of the Reapers. Naturally, this methodical approach limited the ability of turian commanders to take the initiative, despite decentralised control of artillery and air assets. However, this was not and is not considered a problem. The turians do not rely on breakthroughs and seizing lucky chances. Instead, they are utterly relentless, never giving up, using their superior discipline to protect them from excessive casualties.
That is not to say that the turians are so predictable. The number of options they consider before acting is just as large as any human commander would consider. The issue of debate all these decades later is that the turians' strategy had remained utterly static for centuries, and specific strategy regarding a first contact scenario had not been developed. How could the turians have so badly misjudged the capability of humanity to strike back? How did they turn an early victory on Shanxi into a series of embarrassing defeats?
Citadel laws and protocols regarding the same situations were inconsistent and old, a mix of fear generated by the rachni and the hope of finding another species willing to contribute like the turians. Whichever outcome was required or indeed wanted by the initiating species could be legally supported. This is the reason why turians could legally fire on the unknown ships attempting to access Relay 314, even if the humans on board had no idea about Citadel laws. It is also why salarians might not have fired, fearing quite correctly that it would start a new war, and why the asari almost certainly would not have fired, preferring their hopes of a new ally to nightmares of another centuries-long conflict.
That the turians would choose to engage someone flagrantly breaking the law is considered a certainty by historians of all species. The internal crisis of the turian people was reaching new heights just as humanity's rapid expansion began, and the Turian Navy was filled with officers who would rather have fought aliens than their own people. It did not matter who they fought, someone would eventually feel the full wrath of the Hierarchy.
Palaven Command began assuming that the war would be with the Batarian Hegemony, and prepared accordingly.
The strategy they settled on played to their expectations of the batarians, then still a formidable military actor even if they were still only a minor power when all things were taken into account.
In terms of population, the batarians were not far behind the Hierarchy. Turian population growth had long been stunted due to the weight of military service on the younger sections of the populace, and natalist measures were accessible only after retirement at age 30. By contrast, the strong gender roles and emphasis on dynastic pride in batarian society meant unchecked population growth in every sector of society that could afford to have children. For the turians, this meant several things. In ground engagements on the offence, their forces would almost certainly be outnumbered. In terms of military production, the batarians would be able to keep themselves armed for longer than their otherwise meagre economy might suggest. The turians themselves were dependent on other Citadel species to keep them armed, and on the volus for keeping them in good finances to do so.
The turians had no desire to commit genocide. It was viewed as wasteful, and it was likely to be considered criminal by the asari, whom could put the Hierarchy into very dire financial straits in a matter of hours if displeased. This meant that simply dropping asteroids on the tougher batarian colonies, or bombarding them into submission with naval assets, was out of the question. They would have to be taken, a daunting task for any military. The turians did not then possess titans or drop-pods capable of being launched from orbit, making any landing a highly risky venture against protected targets.
Making matters worse was the relay problem. The route followed by the Alliance's Tower Bridge project of 2178 into the Hegemony via the Skyllian Verge was not in reach. The star clusters it started in were not explored by any species until the start of 2158. The Citadel would not authorise exploration. Worse, unlike the three relays that the Alliance could have used to attack the Kite's Nest and its nearby clusters, two were also in unexplored regions, unknown to turian intelligence. At the time, the only relay leading to batarian space was at the 'northern' edge of the Verge, in salarian space. This is the reason why the salarians were able to glean so many contacts among the batarian exiles and expatriates in the Terminus.
There was no question that the Salarian Union would allow the passage of turian fleets to attack the Hegemony, but having only a single relay to attack through was a huge risk. Forcing passage could result in huge losses if the batarians were aware of the attack beforehand, and roaming privateers or wolfpacks could stake out the bottleneck even if the assault was successful. The turians had faced exactly the same circumstances during the Krogan Rebellions. Fanatical krogan pirates had been known to launch near-suicidal attacks against turian convoys as they exited into relay jumpzones. The turians had little doubt that the batarian defence of their home clusters would be equally fanatical.
In response to these particular factors, the turian strategists were able to apply the same moves they had used against the krogan.
If war was declared, or if an incident prompted necessary aggression, turian forces would assault the relay and seize the nearest garden world. Local resistance would be pacified, and the garden world itself turned into bait. Unable to resist due to their fanaticism and patriotism, it was expected that batarian fleets and armies would be immediately dispatched to relieve the seized world, presenting ample opportunity to spring a trap. This would allow the elimination of significant parts of the defending forces, thus easing the way for the next wave of turian attacks to follow up.
Of course, human historians have debated the wisdom of assuming the batarians would indeed flock to retake a colony world they had lost, rather than hiding behind the next set of relays. Such criticisms would indeed be valid if Arch-Hegemon Ar'dra had been in power at the time, but the Hegemony had yet to slip into the hands of a single tyrant. What the Council of Greaters in 2157 would have done in such a scenario is impossible to know, and is alas the subject of great conjecture. Salarian historiography in particular places great doubt on the success of such a gambit, giving the batarian noble castes much credit for their guile.
The strategy was honed for five years through continuous exercises, under the pretext of fighting a colonial rebellion by turian elements. Only commanding officers of cruiser squadrons and above knew that they were in fact practising to fight the Hegemony, those below being told it was a new general strategy.
After Relay 314, it is quite easy to see the part-implementation of this strategy. The seizure of Shanxi was the direct result of it, as was the wait for the human counterattack. However, the turians in direct control of the forces involved were not operating in full conjunction with their own forces back home. They also gravely underestimated the ability of humanity to strike back. This would be their undoing.
STRATEGY
The Systems Alliance
Humanity faced a grave problem as it entered the 2150s, one that was unavoidable and potentially lethal to the entire enterprise of the species. It had never before fought a war in space. In fact, until 2156, it would not face a hostile foe in vacuum, and even then, pirates flying upgunned cargo vessels and shuttles were hardly worthy opponents. The first combat kill by an Alliance pilot is disputed between three officers, one of whom is the famous test pilot and commander of deterrence frigates, Mari Stokke.
As such, even as humanity's fleets were being constructed, it had no idea how to use them. From the foundation of the Systems Alliance, war games and strategic theory were extremely important. The starbase at Arcturus became the centre for this activity. Here, humanity collected her greatest military and scientific minds. Naval and airforce experts from all major powers on Earth, physicists, engineers, and accountants, all the people necessary to shape the new Alliance Navy into something that could deter or defeat foes that would undoubtedly have all the material advantages. Together, they would work out what the Alliance fleets would be composed of, how they would act in battle, how they would manoeuvre on the strategic and operational scales, and how they would interact with battles on planetary bodies. Huge numbers of simulators were built to test the pilots in real time virtual battles, a necessity as many of the ships they would fly had yet to be built.
What the fleets would consist of was decided more or less in the first month. Humanity had the great advantage of discovering the ruins on Mars at the furthest point of technology possible without knowledge and exploitation of the mass effect. What took the asari centuries and the other species decades to use for military applications in space took mere months for humans, not out of any particular racial superiority but rather the lack of readily observable Prothean installations. No doubt if the mass effect had been discovered earlier, the process would have been more difficult.
Most of the vessels would be cruisers or frigates by necessity. The European Union had already designed a highly efficient cruiser in the Geneva-class, more or less a copy of a Prothean ship found wrecked in a hold on Mars. From this, human naval designers scaled up and down. Frigates, heavy frigates, and heavy cruisers were all derived from it. It allowed for the sharing of a great many parts between designs, something that humanity has perfected to an art form since. The 'bow and arrow' shape of the ships would become iconic in the 2150s and early 2160s, until the newer classes were put into service. Only the dreadnought and carrier class designs, the cruiser class ships not scaling well past heavy cruiser tonnages and being unsuitable for use as carriers.
Of course, it is the use of carriers which was humanity's greatest immediate contribution to galactic affairs. There is a great deal of consensus among naval theorists of all species about why humanity was able to conceptualise such craft and why other species could not.
The nature of the other species and their homeworlds vis-a-vis humanity and Earth is the first factor agreed upon. Seventy percent of Earth's surface is covered with water, and it divides the continents in significant ways.
Palaven and Rannoch are both significantly more arid, lacking both large oceans that disconnect the landmasses and forests of trees large enough to facilitate early naval development. This is why both turians and quarians are evolutionary descendants of rock climbing species, and is also why neither had naval traditions to equal their armies' own until after the discovery of the mass effect. Thessia and Sur'kesh equal and almost equal Earth's water coverage, but both have much more equal land distribution, resulting in extensive island chains and mangroves on the former and large forested continents connected by isthmuses on the latter. Quarians and turians never had oceans of a size that would require aircraft carriers. Asari and salarians could always find an island in range of the enemy to park their airforces on, and the asari only rarely engaged in warfare on their homeworld on such a scale after their Technological Enlightenment. By contrast, human nations have needed to fight across the vastness of the Atlantic, Indian, and most of all, the Pacific Ocean.
The military experiences of the various species is the second factor. Not only did humanity have the opportunity afforded by the geography of their homeworld, but the wars fought specifically required the development of the carrier. In wars between European, Asian or African nations, carriers were never an absolute necessity. Just as on Thessia or Sur'Kesh, there was always somewhere close enough to land aircraft. The real problem came when there were hostilities between two powers across the largest ocean, the United States and the Empire of Japan separated by the Pacific Ocean. The Second World War, the largest war in human history before the Cold War, embedded the concept in the human psyche forever. Even countries that did not really require carriers considered them a necessity for any 'blue water navy', a symbol as much of prestige as real power.
By contrast, not even the experiences of the Rachni Wars and the Krogan Rebellions gave any cause for Citadel species to develop carriers. The hardest battles in those conflicts were across relay jumpzones, where it was thought that the dreadnought was the queen of battle, not the fighter-bomber. Dedicated carrier ships for fighters was simply a waste of space, as one could fit a dreadnought in the exact same space and still carry significant numbers of fighters along with it. A relay jumpzone is tiny, relative to the space around it. However, humanity had a good appreciation of this, and upgunned its own carriers with broadside weapons after extensive simulations. Not only could a human carrier make a contested relay jump and survive, but it could then go on to fight dreadnoughts in a broadside engagement on equal terms.
The last factor agreed upon that allowed humans to develop carriers was their attitude towards military risk. Among the Citadel species, only the batarians subscribed to the concept of fighters making independent attacks against anything larger than frigates. The risk of being shot down by GARDIAN batteries was high, particularly as none of the Citadel species had developed EXALT weapons that allowed fighters to engage outside of the range of point defences. As such, the asari completely dismissed the concept as unacceptably wasteful, save for drone attacks. The salarians, turians and quarians used their fighters in direct support of frigates, both to screen them from enemy fighter attack and to boost the power of wolfpack assaults. The Alliance Navy and its sea-going predecessors had little problem with the risks of fighter attacks, having both the weapons to keep the fighters in relative safety and the societal discipline to accept losses.
As such, humanity viewed the carrier as equal in importance to the dreadnought, and the first production runs on both were started at the same time. Their use was wargamed heavily, particularly 2156 and 2157 as the fleet began to take shape in reality. By the time of Shanxi, how they would be used and how their fighter wings would be used was honed to a fine point.
Beyond the fleets was the general plan for First Contact. There were three scenarios, marked Green, Yellow and Red. Green was for the eventuality that friendly aliens would be encountered, and large elements of it would be used in the post-First Contact era for defence of the immediate clusters under humanity's control. Green Plan was largely defensive in nature, with a great emphasis of surveillance and reconnaissance, attempts to detect subterfuge or deception on the part of the 'friendly' aliens being at the fore. Yellow Plan was to be implemented if aliens didn't really care about a new species on the block. This was considered the least likely possibility, but was drawn up after scientists pointed out that aliens could be millions of years ahead of humanity, and could present so large a threat that they might not even bother to attack. Yellow consisted of keeping humanity's efforts confined to uninhabited regions, and cordoning off relay jumpzones to alien sectors.
Red Plan was the one that was implemented ultimately, after the Battle of Relay 314, the Siege of Shanxi and the Battle of Xi'an Valley. It consisted of aggressive, proactive counterattacks, to push the front of the war away from Earth and grab territory with which to negotiate a peace settlement. If no peace settlement was forthcoming, then Plan Black would be implemented, consisting of a relay-by-relay defence all the way back to Sol, designed to sap the strength of any invading force.
No one in the Alliance was under any illusions about the odds. The exploration efforts were stepped up. Relays that led to systems with no inhabitable world were seeded with space stations, Arcturus being the most notable and most important of these. Those with garden worlds were immediately colonised, to set up humanity's legal claim to them. This was arranged by the First and Second Colonisation Congresses, whereby the 'nexus' worlds were shared equally between the major powers of Earth, and the Alliance Colonisation Committee established to divide worlds within clusters outside of relay systems.
Shanxi was the nexus world of one such colonisation cluster, placed under the jurisdiction of the European Union. The EU were granted the largest number of worlds, not only due to possessing the largest population but also due to the wrecked state of Asia in the aftermath of the Cold War. The continent had been in a continuous period of unrest for more than ten years, easing only as citizens began to leave. Shanxi itself was chosen for resettlement of EU-occupied North East China and a smaller number of Polish settlers. Both were victims of the climate and the war, and both needed a fresh start. Located in the 'north-west' sector and having multiple relays in-system, it was always destined to be an important point in early colonisation.
The purpose of this rush was twofold: find and keep the aliens as far from Earth as possible, and grab territory for resource exploitation quickly. In the latter purpose, it succeeded. Eezo was found in more than enough quantities to back construction of ships, and even civilian applications in the mid-2150s. However, it did not keep the aliens away. It drew their attention.