Reservists will protect Llanbedr Airfield and the Trawsfynydd decommissioned nuclear power station from simulated attack from patrols from Malaysia, Japan, Ireland and the US.
The Honourable Artillery Company (HAC) said it was the most ambitious exercise it had ever conducted.
It will be the largest UK ground-based air defence concentration since 2003.
For many of the nations taking part, this will be the first time they will have trained in the UK.
Soldiers from across the world are taking part in a week-long training exercise in Gwynedd from Saturday.