A UN tribunal’s ruling on Tuesday that invalidates China’s territorial claims in the South China Sea confronts Beijing with a pivotal moment in its emergence on the world stage. At issue is nothing less than whether China accepts the primacy of international law — or only does so when it suits the country’s national interest.

The decision by the UN’s Permanent Court of Arbitration in The Hague is binding under the UN Convention on the Law of the Sea (UNCLOS), which defines the rights and responsibilities of nations using the world’s oceans. As a signatory, Beijing should observe the ruling. Failure to do so would damage its credibility in a host of international forums that use treaty agreements to govern behaviour in commerce, the environment, politics and defence.

If China flouts the verdict — which it has already criticised as reckless and illegitimate — it will send a signal to the world that it regards power as the prime arbiter in international affairs. Its defence that, far from being an outlaw, it has actually been trying to safeguard UNCLOS against a rogue tribunal carries little weight. The five judges on the panel reached a unanimous decision and their verdict under the convention is final.