A Force Shield was an energy field produced over structures rendering it invulnerable to attack for a short period of time while the field was active. It was used during the Psychic Dominator Disaster by the Allies, Soviets and Yuri's Army.
The Allies created it as a modification of the Iron curtain blueprints they possibly obtained after the final raid on Moscow. The Soviets and Yuri, having direct access to the Iron curtain project, easily copied the mechanism and incorporated it on their forces respectively.
During the Psychic Dominator Disaster, all factions began to use it to protect vital structures from superweapons. Commanders may access it via a Battle Lab. However, the great amount of power is required to activate a Force Shield, completely powering down the entire base for a minute, and afterwards requiring a long time to recharge. When used properly, it can be a salvation for desperate Commanders.
The Soviet Iron curtain can perform a similar role without draining power from a base, but its effective radius is smaller. The Iron curtain can also cover vehicles and ships, be it friendly or hostile. Skilled Soviet commanders opt to use the Iron curtain first before deploying the Force Shield for maximum protection, if available.
It was not used during the War of the Three Powers. However, the Nanoswarm hive, the redesigned Iron Curtain and the top secret protocols (such as Chrono Rift and Point-Defense Drones) provide a near similar result.