An interesting article by the talented Thomas Hormby at Low End Mac. From the article:
"Mac sales slumped in 1985. Bill Gates, who controlled 2/3 of the Mac software market, didn't want lose his investment in software development, and suggested that Apple license the Macintosh to outside companies. Apple refused, and Windows was released in November, 1985. Apple CEO, John Sculley, was outraged at the Macintosh-inspired software and sued. Though unsuccessful, the lawsuit chilled Apple-Microsoft relations for almost 15 years."

Read More:
The Apple vs. Microsoft GUI Lawsuit