From Halopedia, the Halo wiki
Halogen was a complete conversion mod of Command and Conquer: Generals. Developed by Slipstream Productions, the mod was an attempt to create a real-time strategy version of Halo but was subsequently cancelled following a cease and desist order obtained by Microsoft.
The mod was developed over three years prior to the cancellation. Powered by CnC: Generals's SAGE, the mod featured the traditional assets such as base-building utilities, resource gathering, and unit-management system.
Cease and desist order
Microsoft took notice of the mod's development and obtained a cease and desist order against the production group. On September 9th, 2006, the production group made an official announcement confirming the cancellation of development.
Following the closing of Halogen, the production group recycled the developed assets for a different project titled "Asylum" (later retitled to "Signal").
Halogen and Halo Wars
Rumor had it that Halo Wars was produced directly because of fans' extremely positive reaction to Halogen and the negative response to the cancellation of the project. Halo Wars was announced shortly after Halogen was cancelled, and it was speculated that Halo Wars was designed to feed off of the anticipation Halogen carried. It was later revealed that the reverse was actually true; In the email to SSP from Microsoft, they revealed that the reason they didn't want a free Halo RTS being circulated around the Internet was that they were going to announce a real RTS based on Halo at X06.