U.S. leaders are increasingly worried that China is gaining influence over traditional American allies, as the rising Communist power charts a more aggressive foreign policy course.

Those concerns are primed most recently by the new South Korean president's decision to suspend deployment of a U.S. missile defense system, known as a THAAD. American and South Korean officials agreed in recent years that the THAAD is necessary protection against North Korea, but China fears that the system could undermine the power of its own nuclear arsenal. So they demanded an "immediate" halt to the deployment of the system, backed up by economic measures to hurt the South Korean economy.

South Korea complied, as the newly elected President Moon Jae-in suspended the deployment of the THAAD, citing the need for an environmental review of the program.

Moon's decision spurred concerns in the U.S. "It's my fear that he thinks — I hope I'm wrong — that [Moon] thinks that South Korea has a better chance working with China to contain North Korea than working with the United States," Senate Minority Whip Dick Durbin, D-Ill., told the Washington Examiner.

If Moon comes to regard China as a more reliable partner on his top national security issue — the threat of North Korea — that would represent a significant step away from the traditionally close cooperation between the United States and South Korea. And Moon isn't the only leader in the region to trouble U.S. policymakers. President Rodrigo Duterte of the Philippines spent months last year railing against the Obama administration and calling for a "separation" from the United States.

"China is going to have a heavy hand," predicted Rep. Ted Yoho, R-Fla., who chairs the Committee on Foreign Affairs' subcommittee on the Asia Pacific region. "China is going to want them to do what China says and I think Durbin is correct in that he is seeing Moon do that ... He's trying to appease the Chinese government, and we know how they're going to play. They're not going to play well."

That's not a unanimous view, but even dissidents think it's plausible. "I don't see the Chinese influence in either of those regions substantially reducing our role," said Mark Meadows, a North Carolina Republican who also sits on the Foreign Affairs Committee. "I don't know that I agree with Dick Durbin's doomsday analysis. But it is a possibility. I'm not saying it has no merit. I'm just saying it doesn't pass the threshold at this point."

Heavy-handed influence over South Korea and the Philippines could have significant implications for American national security and economic interests, given China's ambitions in the region. Chinese fighter jets flew hundreds of missions last year into airspace claimed by the Japanese, in an attempt to assert control over the East China Sea. China is also building artificial islands, complete with military installations in the South China Sea. If China accomplishes its objectives in those waters, it would control some of the world's most vital shipping lanes, as well as valuable underwater oil and gas reserves.

Yoho praised President Trump for sending the U.S. warships through the South China Sea on freedom-of-navigation missions that contest China's sovereignty claims. "That's something we should have been doing all along," he told the Washington Examiner.

The Florida Republican acknowledged that other Trump policies had strengthened China's standing in the region, however. The "America First" foreign policy platform created a "false narrative" that Trump would lead the United States into "isolationism," according to Yoho. Trump's decision to pull out of the Trans-Pacific Partnership, a trade agreement with 11 Pacific Rim countries, exacerbated that perception.

"They were counting on TPP and they saw that as a strong message from America," Yoho said. "But it wasn't going to pass. The Democrats weren't going to support it, the majority of them. I wasn't going to support it, being a Republican. And they use that to say, well, we've got to go to China."

Yoho thinks that countries such as the Phillipines might hesitate to oppose China's aggression "in the short-term" — "six months to a year," he suggested — but that trend could be reversed by freedom-of-navigation operations and economic deals with Asia-Pacific countries. "And then the more we do, it builds up our credibility and as we build up our credibility these people say, 'they are back,'" Yoho said.

If that doesn't work, however, and China comes to dominate the region, they would be poised for a military conflict with another U.S. friend. "If you look at what China is doing in the South China Sea ... they're boxing in Taiwan," Yoho warned. "They're definitely prepping for [conflict]."