For most of the 1940's and early 50's (commonly dubbed comics' "Golden Age"), Superman and Captain Marvel were two of the most wildly popular superheroes of the era. In fact, Fawcett Comics' Captain Marvel even surpassed Superman's sales at one point, which DC Comics countered with a copyright infringement suit, even though Cap was never actually that similar to Superman. Nevertheless, Fawcett lost the lawsuit and ceased publication of Captain Marvel in 1953.
In a crazy twist of irony, DC Comics acquired the rights to publish Captain Marvel in 1972, and quickly followed up with a new SHAZAM! series. Instead of integrating Cap into the modern style of comic book storytelling, DC decided to stick with the lighthearted, whimsical approach of the Golden Age stories. In fact, they even hired Cap's original illustrator C. C. Beck to draw the stories.
Despite Superman's strangely-silent presense on the cover of SHAZAM #1 (click on the cover image for a larger view), the two (former) Golden Age rivals had never actually met each other in a comic book story. Their first true meeting would get a little bit closer with Superman #276 (1974), featuring the titanic clash of Superman and Captain Marvel stand-in Captain Thunder!
According to writer Elliot S. Maggin in a 2006 interview, the reason behind using Captain Thunder (despite having the rights to the real deal) was that DC felt the more whimsical Captain Marvel appearing in SHAZAM would be a poor thematic fit with the more "realistic" Superman, and that Cap's appearance in Superman (or vice versa) would be too jarring a transition for either character to make. Whatever the case, we all knew this was Captain Marvel (more or less), and if anyone still had any misgivings about the substitution, this incredible "MUST BUY!" cover by the prolific Nick Cardy was more than enough to overcome them.
I've always been a sucker for "clash covers". You know, the covers where superheroes (or groups of superheroes...or villains) are hurtling toward each other at top speed, frozen in time at that split second before impact. Despite all the "growing" and "maturing" superhero comics have experienced over the past few decades, it's still hard to deny the power and appeal of a good clash cover. Pumping up the tension is, of course, the terrified fleeing crowds below the clashing superheroes, who are seemingly oblivious to the death and destruction the collision of their invulnerable bodies will generate.
In other words, the ideal comic book cover for your inner 10 year-old.
By the way, to see more of Nick Cardy's outstanding DC covers, click here for an extensive (and nicely organzied) cover gallery!