Here is an amazing story about the life or death struggle of 2 major superheros. In the 1940's DC (the creators of Superman) sued Fawcett Publications over copyright infringement with their Captain Marvel title being too similar to Superman. The Big Red Cheese was actually outselling Superman for a few years so this may have spurred DC to take to the courts.
The battle raged on in the courts for 12 years. The ruling was in favor of DC (after an appeal). National agreed to settle with Fawcett out of court, and Fawcett paid National $400,000 in damages and agreed to cease publication of all Captain Marvel-related comics. Being tired of the industry as a whole, they decided to just fold up and close shop on all comic publishing.
Interestingly enough, DC licensed the Fawcett superhero characters in 1972. Captain Marvel was purchased outright in 1980 by DC and is now part of the DC Universe. Superman is still the main hero at DC, but he now has the Big Red Cheese to help him fight his battles!
Check out all the Marvel family of comics before they were part of the DC universe. They are here online: Whiz Comics, Captain Marvel, Captain Marvel Jr, Mary Marvel, The Marvel Family and Americas Greatest Comics.
For more information on this case (which was the longest comic book court battle in history) check out the great article in Wikipedia National Comics Publications v. Fawcett Publications.