Except that's not really true. With comic book titles publishing once a month, and most characters appearing in only one book (in rare exceptions, two), chances were extremely high that even the most devoted Fantastic Four fan would find himself clutching his allowance looking over the new offerings the other three weeks of the month and picking up something different. And in that moment the intrigue of a truly dynamic cover could very well trump the vague notions of publisher loyalty. Forced in a schoolyard debate to pick one company or the other to buy from exclusively for the remainder of eternity, an average kid could settle on Marvel or DC, but those hypotheticals were never binding. Preferring Marvel wasn't automatically equivalent to never, ever reading DC, and vice versa. The only way to identify with one company over the other was to read both.
Because most kids reading comics were at least familiar with the offerings on both sides, Marvel and DC Comics always knew there was a built-in audience eager to hand over money in exchange for intercompany crossovers. Some Iron Man fans might occasionally read a stray issue of The Atom, and some Atom fans might peruse the pages of Iron Man now and then, but put both characters together in the same story in one book, and the draw would be a larger audience than either character could command alone. Factoring in pure novelty, it might even sell more copies than both solo heroes' books combined. For all the analysis of house styles and narrative philosophies, Marvel and DC were primarily interested in making money, and most of the time taking cheap shots at one another in their editorial pages furthered that end, just as most of the time gently urging their existing fans to see partisan support of one company over the other furthered that end. But once in a while, cooperation could make some serious return on investment, as well.
Sometimes this worked out as well in practice as it did in theory. Marvel and DC writers, artists and editors collaborated on a joint project starring Superman and Spider-Man, the respective flagship characters, in 1976. Many other intercompany crossovers would follow, despite the continuing rivalry and competition between the publishers and various logistical hurdles to clear. Those same hurdles had in fact prevented crossovers from happening in the years prior to 1976, which at various times led to peculiar repurposings of completed scripts which became unusable. Such circumstances gave rise to Marvel's Squadron Sinister, on the heels of which came the Squadron Supreme.
The backstory, as briefly as I can condense, went thusly: in the late 60's the writer of Marvel's Avengers and the writer of DC's Justice League of America batted back and forth the possibility of an intercompany crossover featuring the two powerhouse teams, each of which showcased many marquee characters of their respective publishers. Storylines were composed which, in the honored tradition of giving fans what they want, revolved around the two teams joining forces only after first battling one another due to some kind of misunderstanding, thus settling years' worth of childish contemplations as to who would win in a fight, Thor or Green Lantern. The tale was essentially done except for finishing touches when it finally dawned on someone that the legal departments would have to get involved to sort out copyright issues, production costs, profit-sharing and whatnot, all of which ultimately doomed the dream. However, not one to waste the underpinnings of a good yarn, the Avengers writer decided to deliver on deadline a script in which Captain America, Iron Man, Thor and their allies battled a brand new team of villains ... who all happened to have powersets and costumes and codenames extremely reminiscent of Superman, Batman, the Flash and Green Lantern, altered just enough so as not to criminally infringe on the competition's intellectual property. (For what it's worth, the Justice League eventually ended up encountering original characters who were high evocative of certain Avengers, as well, plus many years later DC and Marvel would overcome every obstacle and publish a proper and highly ambitious Avengers and Justice League throwdown.)
Since the analogues of Superman and friends were unveiled in a story as pure antagonists, they were dubbed the Squadron Sinister. The story was on par with most other Marvel adventures circa 1969 and the winking meta-narrative of the Avengers trouncing an evil Justice League doubtless made an indelible impression. So much so that the characters would return again and again, with increasingly convoluted origins (as is the inevitable fate of almost all popular comic book characters) that eventually expanded to include cosmic beings bestowing powers and costumes on Earthling miscreants, but in fact borrowing the inspiration of said powers and costumes from a team of bonafide superheroes inhabiting a parallel Earth. This allowed Marvel creators to have it both ways, and utilize the Squadron Sinister whenever they wanted Marvel heroes to fight DC heroes in the pages of marvel comics, and the Squadron Supreme whenever they wanted both sets of heroes to team up. Placing the Squadron Supreme on an alternate version of Earth allowed them to be kept in reserve for occasional appearances only, dodging the question of why Captain America wouldn't immediately recruit the functional equivalent of Superman or the Flash into the Avengers on a permanent basis. Of course, parallel earths afford creators with other narrative possibilities as well.
DC Comics established its multiverse of various Earths right around the same time that Marvel Comics came into being with Fantastic Four #1. For DC, it was essentially a necessity to explain, for example, how the World War II era versions of the Flash and Green Lantern could co-exist with the Space Age reinterpretations of same, or more to the point, how Superman and Batman could have close to three decades' worth of ongoing adventures and yet never have aged a day. Solution: on Earth-1, Green Lantern was a test pilot and Superman was a young man, while on Earth-2, Green Lantern was a train engineer and Superman was, in fact, getting old. In theory, this allowed every comic DC had published since 1938 to have "really happened", with the caveat that not everything had happened in the same universe. It gets more complicated from there.
Marvel took the idea of a multiverse and ran with it, and immediately took it to more interesting places. While DC required two distinct Earths to explain how a man in red shirt and blue pants could be the Flash and a man in all red could also be the Flash, Marvel didn't require alternate worlds at all, and could utilize the concept with greater freedom and flexibility, creating parallel Earths with wildly divergent histories and unique casts of characters and, most importantly, no narrative limits.
The two paragraphs above are intended to provide some context for a couple of salient points. First, DC Comics eventually made as much creative use of parallel earths as Marvel did, but the end result was an unwieldy network of interconnected ongoing stories that wove back and forth across universes in complicated patterns that even die-hard fans had trouble following. This led DC to publish a maxi-series in 1985 with the grandiose and not entirely hyperbolic title CRISIS ON INFINITE EARTHS. The plotline - involving a malevolent god-like creature, anti-matter, and time travel - literally erased the parallel Earths from existence and rewrote history, thus simplifying and streamlining DC's continuity going forward. In the 80's, most comics were either regular series intended to continue indefinitely, or mini-series intended to run no more than four or six issues to tell a complete story. COIE required twelve issues, hence the newly coined maxi-series label. In retrospect, it was a massively important year in DC's publishing history, and despite numerous imperfections in the execution I am a fairly big fan of it. But during the span it was published, I wasn't reading it in real time. I encountered the odd issue here or there, and I got the idea it was a watershed story that would have major after-effects, but DC was still the other comics company, readable but not as important as Marvel.
Marvel published a maxi-series of its own a year before COIE, an offering entitled MARVEL SUPER-HEROES SECRET WARS. My fond nostalgia for that storyline is limited to my memories of perusing certain issues in my fourth-grade classroom, when we would have rained-out recess indoors. I didn't collect any issues of SECRET WARS at the time because I had been unaware of its existence for the first few months it was being published. And even at the tender age of 9 I was enough of an obsessive completist that if I couldn't have every issue of the limited set of twelve (again, at this time it was all but impossible to find old issues of a comic book series once they had been replaced on the newsstand with newer ones) I didn't want any. Reading someone else's copies was enough for me. And missing the first few issues was no hardship either, because it was as dumbed down as a mega-event could possibly be: good guys and bad guys punched each other for a year on an artificial planet designed to be their ultimate battleground.
But after SECRET WARS, and after CRISIS ON INFINITE EARTHS, there would be other maxi-series, and one in particular would represent a best-of-breed approach: an intelligent story unlike any being told in the monthly status-quo-compliant dustups, published by Marvel but set in another corner of the multiverse than the homeworld of Spider-Man and the Hulk. The Squadron Supreme would get their own twelve-issue limited series starting in the summer of 1985, and I would happen upon its inaugural issue and seize the opportunity to finally collect a complete run from start to finish. Little did I know I would also have my mind thoroughly blown, as I will explain next post.