The game, as it were, takes place in a proxy afterlifeThis was brought on recently by the realization that the (as of this writing) Fall 2013 Event is based on the freaking Naval Battle of Guadalcanal, down to the bombardment of Henderson Field for the final phase of the event. Heck, the third phase (E-3) is even titled Ironbottom Sound, the area between Savo Island and Guadalcanal where many ships the kanmusus are based on met their end. This made me think that what we're really seeing is the spirits of the IJN ships (or maybe the resident kami that Japanese believe to reside in everything) are unknowingly reliving the encounters that got their "bodies" sunk in the real world. Given how many of these ships met violent ends (it was war after all), their spirits are fighting these battles again and again until they let go of whatever grudges they had upon sinking, so that they could move on to the next phase. Which leads to the next whacked out theory...
The Abyssal Fleet the kantai girls are battling aren't simply the "grudges of sunk ships" as implied by the 4koma...They're the grudges of the Allied ships sunk by the IJN during the war. This theory was brought about by the fact that Abyssal Fleet ships use the English system of measurement for their gun calibers (5 inch, 8 inch, etc.), in contrast to the metric system used by the kanmusus (12.7 cm, 41 cm, etc.). If these were kanmusus sunk by careless admirals, as the fandom thinks, then wouldn't they be using the same measuring system? The thing is (and this was after watching documentaries like Discovery channel's Battlefield series), only the calibers of US ships are ever described in this way. Add to the fact that the Fall 2013 seems to be a direct reference to the aforementioned Naval Battles of Guadalcanal (down to starting with night battles, something the IJN mainly did during that campaign), it's easy to put the two together... Which makes one wonder if the monstrous forms of the Abyssal Fleet are simply a type of glamour to make it easier to fight the Abyssal Fleet... instead of fellow shipgirls of the US navy just doing their own missions (like, oh, defending vital airfields), while the US-aligned shipgirls think they're fending off the monstrous forces of the Abyssal Fleet...
Fusing the two above theories; every ship has two parts of their soul. The grudge that they had when they sank, and their original umblemished soul. The grudge will seek out the soul of their original enemy.Somewhere; separated by either oceans or dimensions, there is a fleet made of the souls of cute Allied Ships fighting the grudges of sunken Japanese ships.
The setting is the future of Strike Witches.The Neuroi were Time Travellers as well as space. When they descended on Earth, they stopped World War II from happening. Oh, many of the events were the same because of their engagement; but it was the Neuroi who took the dark karma. Hitler's Time Travel Exemption Act, Godwin's Law of Time Travel. Averted. A planet united, humanity turned to White Magic and the purity of the souls of their children, and destroyed the Neuroi; and drove the shattered remnants of their flying ships into the sea.
Kantai Collection and Arpeggio of Blue Steel takes place in the same universe.More accurately, Kantai Collection takes place after Arpeggio of Blue Steel. Sometimes during or after the war against the Fleet of Fog, humanity managed to either capture or re-create them, putting them under the control of humanity like Iona, as well as developing the technology far enough so that even destroyers and minor ships get to have their own Mental Models as well. Sometimes further after, the new Fleet of Fog, now called kanmusu or shipgirls to avoid the negative connotation of the old name (or the technology has advanced to the point they are different things), gained relatively equal rights with humanity and they are eventually assimilated into human society thanks to their new appearance. It is quite likely that somewhere in between, the nanomaterial used to build the "dummies" of the Fleet of Fog also advanced to the point it can be used to mimic human body perfectly while retaining the same destructive power. The Crossover event in Christmas 2013 suggests that the weapons of ship girls cannot penetrate the Klein Field of the enemy. However, it takes but modifications (presumably to be rather simple, seeing that the entire arsenal can be modified without consuming many resources, and the Abyssal Fleet also get to have them) to make them only slightly less powerful than the Fleet of Fog. Combined with the fact that ship girls are stated by canon to be somewhat human in size, that means that ship girl technology is actually more advanced than the Fleet of Fog despite the objective differences in firepower. This technology is also seen in the form of repair buckets, which can instantly repair Fog ships, as well as Damage Control equipment, which can prevent them from sinking; the Repair Goddess in particular can even restore a Fog ship to full health, fuel and ammo instantaneously, at no additional cost (read: without even using nanomaterials). The Fleet of Fog has no known equipment with anywhere close to similar functions. As for the Abyssal Fleet, they might be The Remnant of the original Fleet of Fog, or other projects that might or might not be the precursors or ship girls.
Kantai Collection and Axis Powers Hetalia takes place in the same universe.Aside from the fact that close to everyone is a personification of some sort (whether ships or countries), it wouldn't be surprising if the Admiral the IJN ship girls answer to is Japan himself.
Kantai Collection takes place in an Alternate History where the "Good Guys" and "Bad Guys" of WW2 have been flipped.In this world, the OTL Axis powers-or at least Japan-are free and democratic and defending themselves against a fascist US and her allies. The Abyssal fleet is the results of American attempts to make their own kanmusu.