Area 51 is a name given to a highly classified American air base in the Nevada desert.
In the plot of Incursion, Area 51 was the site of an engagement between GDI and Nod forces, where the former was attempting to retrieve the remaining Screaming Eagle suit from the vaults. During the fighting, GDI inadvertently activated technology from Red Alert in one of the vaults, including a Chronosphere. This causes a chrono vortex which threw Yuri, who was then an acolyte of Kane and the result of Nod's psychic experiments, back in time. This makes Red Alert 2 a new branch of the timeline.
Area 51 was to also appear in Command & Conquer: Continuum.