The first RIMPAC, held in 1971, involved forces from Australia, Canada, New Zealand, the United Kingdom (UK), and the United States (US). Australia, Canada, and the US have participated in every RIMPAC since then. Other regular participants are Chile, Colombia, France, Indonesia, Japan, Malaysia, the Netherlands, Peru, Singapore, South Korea, and Thailand. The Royal New Zealand Navy was frequently involved until the 1985 ANZUS nuclear ships dispute, but has taken part in recent RIMPACs such as in 2012, 2014, 2016 and 2018.
Several observer nations are usually invited, including China, Ecuador, India, Mexico, the Philippines, and Russia, who became an active participant for the first time in 2012. While not contributing any ships, observer nations are involved in RIMPAC at the strategic level and use the opportunity to prepare for possible full participation in the future.
The United States contingent has included an aircraft carrier strike group, submarines, up to a hundred aircraft and 20,000 Sailors, Marines, Coast Guardsmen and their respective officers. The size of the exercises varies from year to year.
Source: Exercise RIMPAC – Wikipedia