Command & Conquer: Classic Revolution is a mod for Red Alert using the Arda engine.
C&C:CR pitches the USA against the Soviet Union, in a battle for the world. A single resource, Oil, is available from derricks and has to be refined.
It has a more realistic gameplay, so tanks are ways more expensive and only available in higher Tech Levels. Structures can be placed around a construction yard or a headquarters exclusively.
Most units are derived from existing military technology, e.g. M1A1 Abrams Tank, M2 Bradly and Chaparral AA system for the United States, or russian T-72 Tank or a Spandrel missile system.