By Agence France-Presse
The war games, typically involving days of gruelling and sometimes gruesome drills in the Thai jungle, are the largest US-led military exercises in Asia.
They have continued in spite of tensions over a 2014 coup in Thailand, prompting Washington to cut back on military aid and call for a return to democracy.
But the US has also been wary of pushing its long-standing Southeast Asian ally into the embrace of superpower rival China.
This year's 10-day exercise kicks off under a cloud of uncertainty about the role of the US in the region.
It is unclear whether Trump will rollback on his predecessor's much-trailed 'Asia pivot', which banked on boosting security and trade ties as a counterweight to China's growing might.
Trump's administration is also expected to take a softer line on human rights, potentially offering some respite for the Thai junta and other authoritarian governments in the region.
Washington sent the head of the US Pacific Command Admiral Harry Harris to attend the opening of Cobra Gold Tuesday -- the highest ranking officer to visit Thailand since the coup.
Some 3,600 US personnel will descend on Thailand's Chonburi and Nakhon Ratchasima provinces for the war games, along with soldiers from Singapore, Japan, South Korea, Indonesia and Malaysia.
The drills, which have been hosted by the US and Thailand since 1980, have previously involved an array of arduous activities, including a jungle survival challenge requiring US marines to drink snake blood.
US embassy spokeswoman Melissa Sweeney told AFP that America was "eager to resume full cooperation with our valued Thai partners", with some military aid suspended until democracy is restored.
She said this year's drills will focus on humanitarian assistance and disaster relief.
China and Indian will join for those humanitarian exercises, with more than a dozen other countries sending representatives to observe.