SHANGHAI—In the countdown to a legal verdict on China’s sweeping claims to the South China Sea, an increasingly frantic Beijing is mobilizing a diplomatic offensive around three core arguments: that the U.N.-backed tribunal has no legal right to hear the case, that America has instigated all the trouble and that China is the victim.

Pay special attention to the last of these. If, as expected, the panel rules against China there will be a powerful nationalist backlash.

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