The 35th Cobra Gold joint military exercise started with an opening ceremony at the Royal Thai Marine Corps in Sattahip district of Chon Buri province on Tuesday morning, and United States ambassador Glyn Davies calling for an elected government in Thailand.
Twenty-seven countries are taking part, either directly or as observers. A total 8,564 soldiers are involved, with seven countries fully participating - Indonesia, Japan, Malaysia, Singapore, the Republic of Korea, Thailand and the US.
Nine other countries participated in multinational planning - Australia, Bangladesh, Canada, France, England, Italy, Mongolia, Nepal and the Philippines.
Nine other countries sent observers - Brunei, Chile, Laos, Myanmar, the Netherlands, Saudi Arabia, South Africa, the United Arab Emirates and Vietnam.
China and India are taking part in conducting humanitarian civic action projects.
During the opening ceremony with Thai Supreme Commander Sommai Kaotira, US ambassador Davies said Cobra Gold was Southeast Asia's largest multilateral military exercise, and Thailand and the US had co-hosted the drill for 35 years.
"I'm so proud to be here with all of you to open Cobra Gold 2016 - to state and to demonstrate America's unshakeable, longstanding commitment to this region. That commitment is strong and enduring, including here in Thailand, where it transcends any temporary challenges that arise in our partnership," Mr Davies said.
"As deep and broad as our partnership is today, it will grow stronger still when, as the prime minister has affirmed, Thailand returns to elected governance. With a strengthened, sustainable democratic system, Thailand's regional leadership role, and our alliance, can reach its full potential," the US ambassador said.
He mentioned that Thailand and the US had been treaty allies since 1954. Both fought and sacrificed side-by-side on many battlefields and then they worked together to build the peace in Southeast Asia, he said.
"Our partnership has a glorious past and sits on a strong foundation. That foundation allows us to look to the future with confidence. Because it is the future that matters most, for Thailand, for the United Sates, and for Southeast Asia. Cobra Gold is squarely at the centre of those efforts," Mr Davies said.
The military exercise will continue in several provinces in the Central Plains and the East until Feb 19.