I dropped COD roughly after Blops II so trying to understand what the fuck has happened since (especially with their horrendous naming scheme) has been arduous - the games are simply unrecognizable now. For all the faults of the newer battlefields at least the gameplay hasn't changed so drastically - granted, with bf2042, the cosmetic direction has reached a new low, but it's still an appreciable distance from the depths COD has plunged down to.
Do you remember COD Zombies? That mode turned out to take place in the same universe as mainline games, then after the world blew up in BO1/BO2 Zombies we got younger version of the main characters of the original 4 from Shi No Numa traveling across multiverses to stop themselves from destroying the world. This ends up fracturing the timeline, leading to the various "side" games that don't take place in the same universe, like Ghosts/Extinction/Advanced Warfare ect. Eventually there is a primordial dimensional evil that the young 4 have to destroy as much as they have to stop themselves, but it ends up being one huge gayop that two space jannies play in order to keep the multiverse going. If the young 4 don't continue to endlessly hop universes to stop the evil demons from destroying humanity and space time continuum, the reality as we know it will cease to exist so the two have to endlessly fight each other in an endless set of cycles. There is no way to stop it, so once Richtofen first started fucking around with Element 115 all the way in World At War, he doomed the entire COD Multiverse to be endlessly destroyed, over and over again by his own hand, as his younger self has to kill him and try to fix his own mistakes in an endless loop, before the universe restarts all over again in an existential nightmare that only the two space jannies are aware of.
Anyways, the young 4 eventually get tired of this and end up trying to change the cycle, after learning that the two space jannies are actually on the same side, not one being good and one being evil. This leads to them taking the older, insane selves and leading to a rituatlistic suicide pact instead of just killing them like in the previous cycles(without them knowing, of course, they "celebrate their victory" by drinking poisoned wine) This quite literally destroys the entire COD multiverse, by erasing everything that happened, everything that was meant to happen and everybody within it as if they never existed. The only known survivors of that entire timeline are Samantha Maxis, Young Edward Richtofen from another dimension and possibly Samantha Cross from Extinction timeline. This gave us the rebooted COD universe, MW2019 onwards(altho Infinite Warfare is confirmed to be the far future of that new rebooted universe), where these three characters eventually ended up. What remains of the original, previous COD multiverse is now known as "Dark Aether" and is basically a confusing hell dimension where fractured pieces of ruins from every single COD universe and time are mindlessly spread about like toys in a child's room, all the while various demonic creatures from various parallel universes as well as Element 115 abominations like Zombies are the only sentient beings left, with all of humanity in Dark Aether completely extinct. Samantha ends up dying in the rebooted COD multiverse as of "Cold War" anyways, meanwhile Young Richtofen is being set up as a mysterious villain figure in Black Ops 6, probably repeating the same mistakes that doomed the original COD universe. Samantha Cross either never existed or she died a long time ago and the only thing left of her spaceship are her logs and countless cryptids from Extinction Mode and that's all that remains of the original COD universe except for whatever is left in the Dark Aether.
Did you get all that? Good, then you're all caught up, as far as the "story" of Call of Duty goes. BTW nothing that happened in the campaign modes of any of the previous games even matters, it's clear that the Zombies mode is the only one that impacts the universe at large in any capacity. Makarov could have glassed Europe in MW3 and Menendez could wipe out The West in BO2 but in the end, the four time traveling WW2 soldiers from WaW eventually lead to Earth being nuked from a Nazi moon base every time, in every universe, anyways. Kind of puts the entire franchise in perspective, doesn't it?
