The exercise presented a fraught scenario: China's military had blockaded Taiwan by air and sea, and Southeast Asian countries were grappling with how to evacuate as many as 1 million of their nationals trapped on the besieged island.

Over two days in April at a Singapore hotel, some 40 participants and observers in the war game, including serving and retired Asia-Pacific officials and military officers, as well as security scholars, simulated their responses to the unfolding crisis, according to four people familiar with the discussions.

Hours ticked by as some players weighed unified action through the Association of Southeast Asian Nations, while others reached out to the mock U.S., Chinese and Japanese delegates to negotiate special air and sea corridors to extract foreign nationals. Eventually, the people said, a stark conclusion emerged: The Southeast Asian states needed a Singaporean airlift to have a chance of evacuating their people.