Cobra Gold, the largest military exercise in the Asia-Pacific, has been held annually in Thailand for more than three decades, but the United States scaled the drill back this year after sanctioning Bangkok for last year's coup.
"We can't deny that this period is a challenging one and has necessitated a modified Cobra Gold as Thailand manages its return to democracy," U.S. charge d'affaires Patrick Murphy Murphy said in a speech at the opening ceremony for Cobra Gold at a military academy in Nakhon Nayok, east of Bangkok.
The Thai army took control last May saying it needed to restore order after months of political unrest that killed nearly 30 people. The United States responded by freezing US$4.7 million of security-related aid and cancelling some security cooperation.
Comments by a visiting US envoy last month, including that Thailand immediately lift martial law which has been in place since May, further strained ties. Senior Thai ministers responded by telling the US not to meddle in Thailand's political affairs.
Last week Prime Minister Prayuth Chan-ocha, a former army chief, said a general election would take place in 2016 but stopped short of giving a specific date. His military government has said martial law will remain in place indefinitely.
Washington has sent 3,600 troops for this year's exercise, down from 4,300 last year, the US Embassy in Bangkok said in an e-mailed statement.
The exercise comes as Thailand seeks to counterbalance its ties with Washington by cozying up to regional superpower China which says it supports the Thai military government.