Williams Electronics, Inc. v. Artic International, Inc.
685 F.2d 870 (3d Cir. 1982) - full text opinion
Place your cursor over the image to see just how identical Defender and Defense Command are:
This case involves copying the arcade video game Defender, one of the most popular video games of all time. The plot is that you pilot a spaceship trying to stop aliens from abducting humans. It is estimated that people have spent over $1 billion playing the game. With such success, it's not surprising that other companies wanted to replicate the game. Defense Command, however, was not the only Defender-clone. Both Mayday!! and Mirage are also considered to be virtual copies.
You can play Defender online for free, using either Shockwave or Java. As a side note, the man who created Defender, Eugene Jarvis, is still creating video games today, 25 years later.