THE cold war between Africa’s newest neighbours is heating up. South Sudanese troops advanced deep into Sudan on April 10th, capturing its most valuable oilfield, Heglig, in the biggest clash since the south seceded from the north last July. Southern troops claimed to be responding to air and ground attacks from their former master, but the scale of the offensive is unprecedented. A fragile peace process that has survived several bumps in the past few months may now falter. Sudan has suspended its participation in the divorce negotiations in neighbouring Ethiopia. Parliaments in both countries are calling for military mobilisation. The drums of war beat ever louder.

The last straw could be South Sudan claiming Heglig as its own. A ruling by the Permanent Court of Arbitration in The Hague in 2009 appears to put the field in the Sudanese state of Southern Kordofan. But the south now disputes this. “Heglig is deep inside our borders,” says Colonel Philip Aguer, a spokesman for South Sudan’s army, adding that its troops have moved farther north. Sudan will not accept this, and for once it seems to be getting some international support. The African Union is calling on the south to withdraw its soldiers immediately and unconditionally. Sudan has complained to the UN Security Council.

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