A Chinese amphibious invasion of Taiwan would most likely be defeated by a U.S.-led coalition, but not without one key element: Japan’s support.

Just how crucial Tokyo’s role would be was one of the main takeaways from recent war games conducted by the Washington-based Center for Strategic and International Studies think tank simulating 24 different conflict scenarios involving the democratic island.

“In most scenarios, the United States, Taiwan and Japan defeated a conventional amphibious invasion by China and maintained an autonomous Taiwan,” CSIS wrote in a report on the war games.