Over the past five years, President Barack Obama has gained a reputation as a reluctant commander-in-chief. True, he has not shied away from endorsing drone strikes or the killing of Osama bin Laden. But on many questions – such as whether to take military action against the Syrian regime after its use of chemical weapons – he has often appeared indecisive.

This week, however, the president responded firmly and speedily to events in the East China Sea. Last Saturday China announced it was setting up an “ air defence identification zone” around a group of islands that Japan administers but which China claims.On Tuesday the US promptly responded by sending two B-52 bombers into the contested area without notifying the Chinese authorities. The US message was clear: America will resist any attempt to change the status quo over the islands by force.

Nobody doubts that a conflict involving China, Japan and the US over these uninhabited rocks would be disastrous. The stand-off is certainly alarming. On Friday China scrambled combat jets as Japanese fighter aircraft entered the air defence zone. There is a growing sense of a storm gathering over the western Pacific.

But the US was right to respond to China’s initial move in the way it did. The US-Japanese security treaty obliges Washington to defend Japan’s territorial integrity, a guarantee that extends to the disputed islands. At a moment such as this, the US must demonstrate to China that the treaty is robust.

Mr Obama’s move also sends a broader message to US allies in Asia. Many are anxious about Beijing’s aggression. They are also alarmed by Mr Obama’s vacillation on security issues in general. Last week’s intervention by the US was an important signal to them that America, the overwhelming naval power in the region, is engaged.

For the US to sustain this stance against China will be challenging. In the cold war, the US could confront the Soviet Union without fear of economic consequences. The US had no dependence whatsoever on the Soviet economy. But today the US and China are economically intertwined. This makes all US security calculations in Asia much more difficult.

The hope must be that in the next few days a way can be found to de-escalate the tension in the East China Sea. But the fundamental point is that the US needs to stand firmly by Japan. If it fails to do so, it will send a signal to every other nation in the region that the US has become an unreliable ally.

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