Rex Tillerson, made waves internationally on Thursday by suggesting that the US should "send China a clear signal that the island-building stops.
Suggesting China stop its building of artificial islands and militarizing them doesn't sharply break with the policy of President Barack Obama's administration, but suggesting a blockade — or forcefully stopping China from sailing to its land features in the South China Sea — does.
China's response, at first muted, has come back strong, with Chinese media saying that "unless Washington plans to wage a large-scale war in the South China Sea, any other approaches to prevent Chinese access to the islands will be foolish."
"Tillerson had better bone up on nuclear power strategies if he wants to force a big nuclear power to withdraw from its own territories," the Global Times wrote in an editorial.
Bonnie Glaser, a senior adviser for Asia and the director of the China Power Project at the Center for Strategic and International Studies, also questioned Tillerson's depth of knowledge about the South China Sea.
"Some of the things Tillerson said were contradictory," Glaser told Business Insider.
"He was not speaking with notes in front of him, and this is not an issue I think he is very well versed in. He may know oil in the South China Sea, but I'm hearing from some people on the transition team that he misspoke," said Glaser, alluding to Tillerson's time as CEO of the energy giant Exxon Mobil.
Glaser pointed to the more measured statements on the South China Sea before the Senate Armed Services Committee from retired Gen. James Mattis, Trump's defense secretary nominee, as evidence that Tillerson went too far.
"The bottom line is the international waters are international waters, and we have got to figure out how do we deal with holding on to the kind of rules that we have made over many years," Mattis said on Thursday.
Tillerson seems to want to stop China from operating in international waters.
And his testimony contained a major contradiction, Glaser said.
"Tillerson did say that there would not be any change to the US position on recognizing China's sovereignty on land features in the South China Sea," Glaser told Business Insider. "If we don't object to China's land claims, do we have a legal right to deny China access to its sovereign territory?"
Furthermore, if the US tried to blockade China from the islands in the South China Sea, "that position would result in conflict," Glaser said.
If the US were to place "a cordon of ships around one or all of the islands, and the Chinese flew in aircraft to one of their new islands, what are we going to do? Shoot it down?" Glaser said. "We'd certainly end up in a shooting war with China."
However, some legal experts side with Tillerson. In a piece published Thursday in Lawfare, James Kraska of the Naval War College wrote this:
"China's interference with US warships and military aircraft in the South China Sea constitute a breach of its legal obligations under UNCLOS (United Nations Convention on the Law of the Sea) and customary international law and are internationally wrongful acts within the law of state responsibility. In such law, injured states are entitled to take lawful countermeasures to induce compliance, such as withholding recognition of China's right to freedom of navigation and overflight in the South China Sea to block access to its islands."
So while some legal basis may justify a huge US naval presence in the South China Sea blocking Beijing from its claimed islands, experts on the US and Chinese sides agree that such a measure could mean war between advanced world powers with nuclear capabilities.
Business Insider reached out to the Trump transition team about Tillerson's comments on the South China Sea and will update this story with any response.