Foothold situation

From The Stargate Omnipedia

An emergency situation within the S.G.C., where an alien incursion has taken over the Cheyenne Mountain complex, and is using it as a beachhead for further conquest.

A simulation of this type of scenario is used to test potential new entrants into the Stargate Command operational staff.

KEY EPISODES

Foothold - Samantha Carter informs Maybourne that there is a foothold situation at the S.G.C.
Proving Ground - A simulated foothold situation is used to test the leadership qualities of Elliot, a lieutenant fresh out of the military academy.
Reckoning, Part 2 - General O'Neill orders an evacuation and lockdown of Cheyenne Mountain after Replicators penetrate Stargate Command.