The OS-Tan Wars refers to a series of battles between different OS-Tan Factions, mainly analogous on the market competition of the mid-1990's. However, it can be said that the OS-Tan Wars extend as far back as 1981, when a computer's operating system rivaled its hardware in importance as a benchmark.
Although at the time, battles occurred between nearly every faction (e.g., Commodore vs Atari), the main competitors were Microsoft, sponsored by the industry giant IBM, and Apple. Using IBM's corporate mass, Microsoft was able to grab a large portion of PC users early on, giving them an advantage in latter battles. After the release of Windows 1.0, IBM expected Microsoft to aid in the development of OS/2 as agreed. Development on OS/2 was slower than expected, and produced an amazing, but rather buggy v2.0. Microsoft decided it was in it's own best interest to abandon IBM and the OS/2 project for it's own landmark OS, Windows 3.1. The OS/2 project was further harmed when IBM executives secretly sent employees to help aid the development of Windows NT. From 1985-1995, while Apple began to flounder under the rule of John Scully, Microsoft steadily gained ground against Apple.
In 1995, Microsoft presented it's foremost challenge to Apple, Windows 95. By uniting MS-DOS with it's graphical counterpart, Microsoft was able to drive most other DOS-compatible Operating Systems out of the market. Compared to Apple's System 7, Windows 95 was much better marketed, and secured Microsoft's position as provider of the most commonly used consumer OS. In addition, they capitalized on the fallout of the Unix Wars by deploying Windows NT to service the gaps created in the workstation and server markets.
The years that followed can be considered relatively quiet, as Apple struggled to keep afloat against the flood of Windows PC's. The late 90's, however, showed some action return to the OS arena. With the return of Steve Jobs to Apple, and the introduction of the iMac (now called iFruit) and Mac OS X, Apple began to gain market share. Linux, the open-source OS anomaly, also began to gain greater popularity, especially in underdeveloped and developed markets previously ignored by Microsoft. Now Microsoft is under heavy assault from all sides, in terms of quality, price, security and accessibility. In this sense, the future cannot be reliably predicted, and perhaps there may never be a conclusion to the OS-tan War.