Over the past 10 years, Washington's interests have clearly shifted from Thailand to the Middle East and elsewhere, said Benjamin Zawacki, a visiting fellow from Harvard Law School.
This lack of attention from the US has enabled China to gradually gain ground in Thailand by increasing its presence in the region, he said.
Thailand is important to the US, but that country's role of being the guardian of democratic principles is losing ground to China's pragmatism, he said.
While the US remains undecided on whether to impose full sanctions on Thailand or support the military government, China has curried favour with the junta by actively supporting it, he said.
Thammasat University historian Thanet Aphornsuvan said: "The current political regime in Thailand has led the country to align with the Chinese."
He added that US Assistant Secretary of State Daniel Russel's recent comments had seriously affected how the military government felt about the United States.
Thanet said the US had a clear economic interest in Asia and, therefore, had to deal with Thailand.
The Trans Pacific Partnership, a comprehensive free-trade pact, remained a big issue that the US had to discuss with Thailand. This country so far has not participated in the talks.
The US would feel better about dealing with an elected government and not a military regime that favoured China more, Thanet said.
In Washington, US State Department spokeswoman Jen Psaki said Russel, who visited Thailand on Monday, and US Embassy charge d'affaires Patrick Murphy, who met with Deputy Foreign Minister Don Pramudwinai on Wednesday, had simply reiterated the United States' stance on Thai politics.
Washington expressed the hope that it would continue to have a dialogue with the Kingdom, she said.
"Obviously, Thailand is a valued friend and ally, and we have a range of conversations with them," she said.
Despite the diplomatic tension, the US will not change any cooperation with Thailand, notably the joint military drill Cobra Gold, but the drill will be scaled down and refocused, she said.
"This year, Cobra Gold will focus on humanitarian assistance and disaster relief with the aim of expanding regional cooperation and coordination in these vital areas," she said.
The drill will take place from February 9-20 with the participation and observation of 26 countries in the region.