Good timing for this thread to appear. In the last week or so, we've been having on-and-off discussions about various aspects of WC in the CIC Discord.
I don't think the US-Japan comparison entirely fits. Parts of it can certainly be applied, but not all of it. It's kind of like it's all of those comparisons rolled into one.
Action Stations has a brief part about the differences between the Kilrathi and Confederation:
Action Stations said:
But it was the world they had taken with the colonists that troubled Vakka. Normally he would have killed them out of hand, but there was something different about these aliens, and curiosity compelled him to keep them alive. From them he had learned much, not in the manner of the Emperor's official "questioners," who had tortured human captives for information, but rather by simply talking. He found that in many ways he liked these aliens, but even more, he feared them; a fact he could never admit before this gathering.
"We have learned this, at least," Vakka finally replied. "If they have an advantage it is in their depth, their web of alliances with half a dozen races, the sheer number of worlds they have colonized. Such a depth of organization could be of infinite help if the challenge from within the core is to one day be met. We lack that depth. We annihilate or enslave everyone on the worlds we take."
"So?" the Crown Prince replied, his tone obviously conveying total confusion over the intent of Vakka's statement.
"Yes, we have a fleet, the best in the galaxy, but we don't have the infrastructure, the web of commerce. We conquer, destroy, populate a new world like a fiefdom, placing a few tens of thousands of our own blood where billions once existed. Those whom we suffer to live, labor in our factories as slaves, not allies. Then we expand yet again. We are like a hollow shell, the Confederation is a solid mass."
Now, part of the description is clearly referencing the differences between the US and Empire of Japan. The "Greater East Asian Co-Prosperity Sphere" really meant the domination of the Japanese over the rest of Asia. Their conquered regions were not willing aids in the war effort and the few replaced the many in those locations unless they possessed particular knowledge of value to the conqueror.
I'm not so sure either side ever has a massive technical advantage outside of the initial strikes at McAuliffe where the Kilrathi have torpedoes and Confed does not....but will get the tech from defective weapons (slave laborers really don't care about quality control). Stealth tech is certainly a Kilrathi edge until the very end of the war but not one that has many serious effects outside of the loss of the
Tiger's Claw.
Forstchen makes it a bit of a jumble because he's clearly calling on those various comparisons to describe the Kilrathi and Confederation. Things like the Hakaga shipyards, deep in the rear of the Empire, is much like the Soviets moving their factories to the Urals to avoid their destruction. But in
End Run, Forstchen makes the Empire more like the Japanese (not protecting their logistics, shipyards, etc because it was not seen as "worthy duty") because the Japanese never developed a serious convoy system or effective anti-submarine measures (like how the Kilrathi don't start protecting their logistical areas until after Confed starts smashing all of them in 2667-68).
So, I'm not sure if any of those comparisons really work because, if anything, it's more like the US v USSR. Two very powerful countries, with large areas of control, supplies, manpower, etc that if they went to war (sans nukes) it would be a grappling match of two almost evenly matched opponents until one starts to break. Things do seem to change in the mid-2660s. 2666 is a very bad year for Confed. They lose at least eight fleet/heavy carriers (ER says half the fleet and then lists 8 other carriers) during the year. That's part of the reason for the CVE program, to help replace losses, but also to do deep strikes against targets. During 2667, Confed starts to bounce back and really take the Empire to the cleaners (Raid on Kilrah, Vukar Tag, other CVE raids) but then the False Armistice occurs and that greatly weakens Confed. That's followed on by the Earth Defense Campaign where Confed loses a number of ships and pilots. That's why they are so desperate in 2669 to put the
Victory on the front lines and turn to weapons like the
Behemoth and T-Bomb to end the war quickly before the Kilrathi launch their reformed fleet (they also took a beating during BoT) at Sol again. Where, had they done so in 2669, it seems fairly clear Confed would have resisted but ultimately would have lost the war.
Returning to the original question, I think it's a little bit of everything for the comparisons. If I was weighting the two sides, certainly the Kilrathi are more like the Soviets and Japanese in terms of how they treat their soldiers, sailors, marines, etc. And Confed is more representative of the Western Allies.