Over 160 countries and the European Union have signed on to the United Nations Convention on the Law of the Sea (Unclos). The United States, however, has long declined to do so.

Now, China has indicated that it might exit the convention if an upcoming ruling by an international tribunal runs counter to its questionable position: that nearly the entire South China Sea is its territory.

The US is urging China to respect the upcoming ruling, which could happen this month and is widely expected to favor the Philippines. In 2013 the Philippines petitioned an international tribunal—the Permanent Court of Arbitration, in The Hague—to rule on whether China’s “nine-dash line” (see below) is valid, under Unclos.

US secretary of defense Ash Carter, speaking at the Shangri-La Dialogue earlier this month, stressed the importance of China respecting the tribunal’s ruling:

The United States views the upcoming ruling by the UN Arbitral Tribunal on the South China Sea as an opportunity for China and the rest of the region to recommit to a principled future, to renewed diplomacy, and to lowering tensions, rather than raising them. All of us should come together to ensure this opportunity is realized.

That position is undercut, of course, by the US’s own failure to join Unclos.

Despite efforts by the George W. Bush and Obama administrations, and support from the US business community, environmental groups, and the military, the US Senate has never ratified the convention. Ratification has been blocked for years by a few conservative Republican senators, with Oklahoma’s James Inhofe playing a prominent role. Many US conservatives consider involvement in some international organizations and treaties as detrimental to national interests, and as an infringement on national sovereignty.

While she was secretary of state, Hillary Clinton said in 2012 that opposition to the treaty was “based in ideology and mythology, not in facts, evidence, or the consequences of our continuing failure to accede to the treaty.”

In 2009, China officially submitted (pdf) its nine-dash-line map to the United Nations. The map shows most of the South China Sea being enclosed by the line. The accompanying text included this passage:

China has indisputable sovereignty over the islands in the South China Sea and the adjacent waters, and enjoys sovereign rights and jurisdiction over the relevant waters as well as the seabed and subsoil thereof (see attached map).

Vietnam, Indonesia, and the Philippines subsequently objected to it. They asserted, among other things, that China’s claims, as reflected in the map, are without basis under the convention. They noted the line overlaps with the exclusive economic zones (EEZs) of other nations. Under Unclos, nations have exclusive rights to exploiting resources in their EEZs, which extend 200 nautical miles from the shore.

Some worry that China might become more aggressive after a ruling that doesn’t go its way. Indeed, almost immediately after the Philippines filed the case with the tribunal, China began setting about building, and later militarizing, artificial islands from which it could project its emerging maritime power.

In a show of strength of its own, the US Navy recently deployed not one but two aircraft carrier strike groups to the South China Sea—a rare move.

Now the US, while trying to tell China to abide by the international law, faces a credibility gap. If China leaves the convention, as it’s now threatened to do, there’s little the US can say about it.