Dawn of the Tiberium Age (DTA) is a stand-alone mod that combines Tiberian Dawn (C&C 95) and Red Alert using the Tiberian Sun engine. The project's aim is to allow people to both play and mod TD and RA, while having all of the benefits of the TS engine.
DTA's classic mode aims to mimic TD's gameplay as closely as possible, while the enhanced mode adds many new features to the TD factions, adds balance improvements and also allows you to play as the RA factions. In other words, you're able to for example have matches between Nod and Soviet or mix it up however you like.
DTA comes with a large amount of content, including 22 singleplayer missions, 93 multiplayer maps and 10 co-op missions. DTA contains many significant technical improvements compared to Tiberian Sun, including support for the latest Windows operating systems and the client that allows the player to define starting locations and teams in the game lobby prior to starting a match. Multiplayer is handled through the community-hosted CnCNet service.
The mod is still actively being worked on, so you can expect frequent updates and if you find any bugs, please report them so that they can be fixed as soon as possible.
The mod is downloadable and support is available on its homepage at ModDB: http://www.moddb.com/mods/the-dawn-of-the-tiberium-age