Some 500 troops were flown in this week to rehearse an imaginary humanitarian-relief crisis, The Miami Herald reported Sunday.
Soldiers, sailors and Homeland Security officials are part of the every-other-year drill, spawned in the 1990s after tens of thousands of Haitians and Cubans overwhelmed the base.
The exact scenario is classified and officials won't say whether an actual event would be created by political unrest or natural disaster. Col. Greg Julian, spokesman for the U.S. Southern Command, would say only the humanitarian war game is about a "mass migration event" in the Caribbean.
Among other things, the troops will be working to register 1,000 migrants a day.
The International Organization for Migration is taking part, but not the International Committee for the Red Cross.
The exercise was planned long before Cuba last month dropped its policy of requiring citizens to get exit visas to legally leave the island.