When last we left off, the Silver Age was coming to a close with the death of Gwen Stacy. The actual end was precipitated, however, a little bit before, with a two-part Spider-Man story involving the standard "drugs are bad" lesson — a story the Comics Code Authority refused to authorized because they forbade any instance of drugs. Stan Lee, being the Man, printed it without the Code's approval and it did great, so the Code ended up changing and losing a lot of its power.
What does this mean for the Bronze Age? Well, darker themes, more horror-type comics (like Tomb of Dracula), and an emergence of more socially relevant stories. But what does this mean for crossovers? Not much. The big thing that affected Marvel crossovers didn't come from the Comics Code...it came from licensing.
In the mid-1970s, Marvel began licensing a ton of different properties. They hit it big with Conan and Star Wars, but they also licensed other properties, including Shogun, Battlestar Galactica, Doctor Who, Logan's Run, Godzilla, and even 2001: A Space Odyssey — written and penciled by Jack "King" Kirby himself. That's where he introduced Machine Man (then called "Mister Machine")
As soon as they started printed licensed comics, they started including crossovers from already established Marvel heroes. Godzilla fought the Champions and then the Avengers. The Master of Kung-Fu (a new character licensed from Fu Manchu) fought against the Man-Thing. Thanos and Drax the Destroyer even had a backup story in Logan's Run #6 (June 1977).
The crossovers didn't end with licensed characters, though: one of the biggest was the intercompany crossovers, Superman vs The Amazing Spider-Man: The Battle of the Century (January 1976). This was a giant of a crossover: it began with Superman defeating Lex Luthor and Spider-Man defeating Doc Ock and them both being sent to, you guessed it, the same prison. The villains escape and start to manipulate the heroes into fighting one another (even shooting Spider-Man with some red sun radiation to even their powers up).
Not every crossover was this epic, however. To capitalize on their most popular character, Marvel started publishing Marvel Team-Up, a book where Spider-Man would team up with various other heroes. Sometimes, this would make a lot of sense, like teaming up with the Human Torch or the Black Widow. Other times...he would team up with the Not Ready For Prime Time Players from Saturday Night Live (Issue #74 in October 1978) or King Kull (Issue #112 in December 1981). And then there's the case of Marvel Team-Up #137 (January 1984), which teamed up Spider-Man and Franklin Richards and saw Aunt May become a herald of Galactus called the "Golden Oldie." I kid you not. (If this seems odd, just read the synopsis: "Aunt May becomes the new Herald of Galactus; She and Franklin Richards seek out the universe's largest Twinkie.")
Marvel also started publishing a similar book starring the Thing called Marvel Two-in-One. This series saw the Thing team up with various characters like the Man-Thing and the Guardians of the Galaxy (the ones from the year 3000). And again, sometimes the crossovers were fairly strange. In Marvel Two-in-One #47 (January 1979), the Thing teams up with the Yancy Street Gang to fight faceless robots and in Marvel Two-in-One #99 (May 1983), the Thing teamed up with a licensed hero, ROM (these adventures and many more brought to you by the amazing Bill Mantlo). In fact, the 100th issue (June 1983), saw the Thing team up with an alternate reality version of himself where he never became the Thing!
In June 1982, however, Marvel did something a bit different: they published a three-issue limited series specifically designed as a way to bring together a ton of different Marvel characters together to fight each other. It was called Marvel Super Hero Contest of Champions and it was the very first limited series crisis crossover. It was about the Grandmaster challenging the "Unknown" (later revealed as Death) to a game, with the stakes being the life of fellow Elder of the Universe the Collector. The two gathered forth their teams of heroes, including a slew of new "international" characters, like Talisman (an Australian aboriginal), Blitzkrieg (a German with electrical powers), Le Peregrine (a Frenchman), and, of course, Shamrock (a Irish superheroine with luck-based powers).
Other crossovers after this included the "Casket of Ancient Winters" (where the Casket was opened in The Mighty Thor #348 (October 1984) and nearly every other comic had characters wondering why it was suddenly snowing) and the "Wraith War" (Avengers #244-245, Incredible Hulk #296, Uncanny X-Men #185-188, ROM #52-65) where various characters must deal with the invasion of Earth by the Dire Wraiths.
And then, in 1984, Marvel made a deal with Mattel to create a new toyline featuring their characters. I told you licensing was important to crossovers!
Marvel quickly pushed through their twelve issue mini-series called Marvel Super Heroes Secret Wars from May 1984 to April 1985. It was, in fact, so rushed that it was filled with continuity errors (Doctor Doom was actually dead at the time, but they didn't have a chance to change that before the series came out, so he just appears without any explanation), but Marvel did do something innovative for their first gigantic crossover: they had each book involved skip ahead to afterwards. So in Amazing Spider-Man, Spider-Man was already wearing his black suit before he even found it in Secret Wars. And in Fantastic Four, the Thing was replaced with She-Hulk and readers had to wait until the end of Secret Wars to find out why!
The mini-series sold exceptionally well. In fact, it sold so well, Marvel decided to repeat it again next year.
And thus the Bronze Age ended.
And so begins the Dark Age...