Humans vs. Humans
Terrans
Former convicts who managed to create a society of their own. Having found themselves in the Koprulu Sector, they fought among themselves, survived the invasion of the Zerg and their fellow humans from Earth. And in the end, they earned the right to exist.
Advanced technologies allow the Terrans to clone people, rapidly terraform planets, create powerful war machines, relocate their bases, and control the Zerg. In Starcraft 2, the race achieved many scientific breakthroughs that affected its military. But the Dominion fleet, the main Terran faction, was weakened.
A major role in the story was played by the United Earth Directorate (UED). A single expeditionary fleet proved enough to establish law and order in the sector for a time. Then Blizzard decided that fans were more interested in watching the romance between Jim Raynor and Sarah Kerrigan than learning the fate of humanity on Earth. Therefore, the UED will not take part in the conflict.
Terran Forces
The main strength of the Terrans is their battlecruisers: starships that transport troops and bombard planets. Strong armor and laser batteries allow them to perform well in combat against ground and air enemies, while the "Yamato" cannon easily destroys armored targets. And jump drives allow them to enter and leave battle effectively. Not to mention that battlecruisers do not have to travel through the Warp, unlike the Imperium's fleet.
Terran aircraft are light, maneuverable, and very vulnerable. Cloaking fields allow "Banshees" and "Wraiths" to carry out devastating raids on enemy bases. They provide good cover for transports that unload troops behind enemy lines.
On the ground, Terrans build a strong defense. Bunkers, tanks, and missile turrets with "Vikings" destroy most enemies on the approach. Automated systems handle defense without human involvement. But if one link weakens, all the others will be put at risk.
The backbone of the army consists of infantry. "Marauders" destroy armored targets, while "Reapers" carry out sabotage. Advanced technologies make it possible to equip an entire army of marines with powered armor that is not much inferior in durability to Space Marine armor. Gauss rifles penetrate the powered armor of other marines, and stimulants increase the fighters' effectiveness.
The life expectancy of marines on the battlefield is measured in minutes, and sometimes even seconds. It may even seem that they are weak and defenseless. But against opponents like the Protoss and the Zerg, surviving is difficult even in very good armor. For comparison, one may recall Space Marines in Terminator armor, who are easily torn apart by Genestealers' claws. And Terminator armor is vastly tougher.
The ace up their sleeve is the "Ghosts" — specialists with psionic abilities that make them superhuman. Ghosts excel at sabotage and guide nuclear missiles onto enemy bases.
The psionic abilities of Ghosts are inferior to those of powerful psykers, but they are not connected to the Warp. They do not have to literally make contact with Hell in order to use their powers. Terran psionics need not fear side effects such as mutations, hallucinations, voices in their heads, turning into a vessel for a daemon, or a portal for hordes of daemons.
Command
The Terrans are in bad shape when it comes to leadership. The logistics genius and charismatic leader Arcturus Mengsk was killed. A similar fate befell the competent general Edmund Duke. And the freedom fighter and experienced saboteur Jim Raynor is living happily ever after with Sarah Kerrigan.
Valerian Mengsk, who commands the main Terran forces, became famous for losing almost half of the Dominion fleet during the attack on Char. He knew perfectly well about the Zerg's characteristics and the resistance awaiting him. And yet he was still caught off guard by an attack from the planet.
In Starcraft 1, humans knew much less about the Zerg — most considered them dangerous animals. That did not stop General Duke from successfully invading Char to cause trouble for the Overmind. Although his forces suffered significant losses, Duke achieved more with fewer forces — without using an artifact that burns the Zerg en masse.
Problems with Machines and Daemons
In the world of Warhammer 40,000, the Terrans may lose before the battle even begins. In the grim galaxy of the future, AI loves to rise up against its masters. For that reason, it was nicknamed the "Abominable Intelligence." The Terrans rely heavily on AI. And if it decides to rebel, its masters will have a very hard time.
There will also be problems with Chaos. Corrupting forces use many tricks to gain new servants. The inhabitants of the Imperium have had resistance to temptation cultivated in them for a long time, and even then specialists still have to carry out regular purges. Unprotected Terrans have a high chance of becoming cultists who sacrifice themselves in hordes for their new masters.
To prevent that, it is worth assuming that the Terrans have a weak presence in the Warp. They would be like the Tau, who use AI and are resistant to Chaos. In exchange, the Imperium's problem with slow responses to threats would be removed — a crusade would be organized immediately.
Imperium of Mankind
An empire stretched across almost the entire galaxy. It is a cruel society where human rights are violated, technology is degrading, and the bureaucratic system is unwieldy. After the Emperor's ascension to the Golden Throne, the Imperium has spent more than 10,000 years fending off attacks by heretics, mutants, and xenos (aliens) from all directions. Humanity has proven that even in such a condition it is capable of surviving and standing up for itself!
The Imperium's Army
The Imperium's fleet consists of cathedrals flying in space. They are inhabited by hundreds of thousands of people. And every one of them hates the enemy with all their soul. In battle, void shields allow ships to withstand significant damage and fire all their weapons. Many admirals love sending boarding parties and ramming an inattentive enemy.
Space Marines: superhumans with a century or even a millennium of combat experience. Their arsenal and vehicles allow them to destroy any foe in close-quarters and ranged combat. And mighty Librarians are capable of protecting allies and destroying enemies by force of will alone. Their ability to drop from orbit puts any concentration of enemies at risk.
Imperial Guard (IG): the best soldiers selected from the Planetary Defence Forces. They possess a vast arsenal of simple but effective weapons and vehicles, and use their own psykers. Their artillery is capable of destroying entire fortified lines — exactly what happened during the Siege of Vraks. Their main weakness lies in communications equipment — not everyone gets a radio, let alone comm-beads.
Adeptus Mechanicus: loyal servants of the Omnissiah (the Machine God). Humans who despise the weakness of flesh and replace their bodies with machinery. The Mechanicus and their loyal Skitarii servants are tougher, faster, and more enduring than ordinary humans. They are terrifying in both melee and ranged combat. It is the Mechanicus who are responsible for creating and repairing the engines of war.
The weakness of the Imperium's army is its fragmentation. There is a reason for that: if part of the IG falls to heresy, the others must deal with the problem. The Mechanicus are interested in their own goals, and the Space Marines rarely condescend to concern themselves with the problems of mortals.
The War Between the Imperium and the Terrans
The Terrans will have time to prepare. They will easily adapt to the new realities, begin gathering information, and prepare for war. But expansion into the Imperium's territory will bring problems — even if they manage to capture a small hive world, they will have to deal with billions of fanatics among the local population.
Years later, a small crusade will arrive in the sector. It will consist of 15 IG regiments and a Mechanicus expeditionary fleet. The Space Marines will send 300 battle-brothers each from the Ultramarines and the Blood Ravens chapters (of course, they had to be included). The Ultramarines will be led by Captain Titus from the game Space Marine.
Among the IG soldiers, the hero of the books, Commissar Ciaphas Cain — Hero of the Imperium — will accidentally turn up, having once again "returned from the dead." Cain considers himself a coward, but that does not stop him from taking on the most dangerous missions. The bureaucrats from the Administratum became so tired of repeatedly removing him from the rolls of the dead that now the hero is eternally "on duty" — even after his honorable funeral. The mere presence of the commissar is enough to inspire soldiers to heroic deeds — a fact the commissar himself is unaware of.
At the beginning of the war, the advantage will belong to the Terrans. In space and on planetary surfaces, they will deliver painful blows to logistics lines. Ground and air stealth units will become a true nightmare for key facilities and command structures. Thanks to superior armor quality, maneuverability, and communications, the marines will be able to toy with the IG. Only the presence of the legendary commissar will allow the fighters to preserve their morale and continue fighting.
The Terrans cannot win in an open battle. But with pinpoint strikes and sabotage, they may slow down or even halt the crusade. A stalemate, in which neither side can advance, would count as a victory for the Terrans.
The Imperium will win if Valerian once again decides to send the fleet into a frontal attack. The power and maneuverability of battlecruisers are impressive, but they cannot compare to the power and variety of the Imperium's fleet.
Having achieved supremacy in space, the fleet will be able to conduct orbital bombardments and cover deploying forces. The IG's artillery will begin working over fortifications, while the Space Marines will start dropping into the middle of enemy concentrations. Captain Titus alone will be enough to destroy a small army! They will also deal with stealth units: the Imperium has extensive experience fighting the Tau and the Aeldari.
The Terran army will be divided and crushed. The Adeptus Mechanicus will destroy technologies tied to AI, and confiscate the rest for secret experiments. The remaining Terrans will be spared from annihilation by the ever-present Holy Inquisition — the inquisitors will do everything possible to preserve humans resistant to Chaos. As a result, the Terrans will survive, but they will lose their freedom and be forced to embrace faith in the Emperor.
Analysis
The Terrans are brave, cunning, and inventive. They adapt quickly to new threats, and their technology is constantly developing. They are capable of causing trouble even for such a monster as the Imperium of Mankind. But that is where their capabilities end.
There are far too few Terrans for them to pose even a minor threat. If Sisters of Battle, Primaris, Imperial Knights, Titans, Grey Knights, assassins from several Officio Assassinorum temples were added to the crusade, and the use of Exterminatus were permitted, then an interesting battle would turn into a one-sided slaughter.
The war might have unfolded differently if Blizzard had developed its universe the way GW did. The UED is an interesting and powerful faction that would have made the conflict with the Imperium larger in scale and more dynamic. Alas, it was not given the attention it deserved.
