In 2006 I briefed David Cameron before a visit to Darfur. On his return he was indignant: "This is ethnic cleansing and we cannot remain silent in the face of this horror." That was in opposition. Now that he has the power to influence events in Sudan, his silence is deafening.

The UN resolution passed last Wednesday will do nothing to avert war in Sudan. It threatens sanctions against both Sudan and South Sudan if violence continues. But the latest resolution is a smokescreen. It creates an illusion that the UN is a united front exploiting every diplomatic effort in Sudan. If that were true, why not implement any of the 16 existing resolutions against the oil-rich Sudanese government?

Sudan's leadership in Khartoum refused a UN request to give aid to its own citizens in South Kordofan. Not surprising given the food shortages are a direct result of the regime's bombardment of civilian lands. Their crime? Sharing southern ethnicity. Why did the UN not invoke the responsibility to protect mandate to ensure aid reached civilians? Why have sanctions never been imposed for Khartoum's flouting of no-fly zones and arms embargoes? Despite being indicted by The Hague for genocide, Sudan's President Bashir is still met by diplomats as though he were a credible partner in peace.

When the Comprehensive Peace Agreement (CPA) was signed in 2005, it was hailed as a diplomatic triumph, marking the end to a 20-year civil war. But it was doomed to fail, not least because it neglected to deal with disputed oil-rich border regions. It is Bashir's exploitation of border uncertainty that triggered recent events. South Sudan's president, Salva Kiir, had fended off Sudanese bombardments for weeks. In the process of driving the aggressors north, Kiir temporarily seized the oil hub of Heglig, which is in a contested border area. He bowed to international pressure and withdrew on condition that the UN would create a buffer zone to prevent further attacks on Southern civilians. There is still no buffer zone and Khartoum continued to bomb civilians.

In May 2011, Khartoum seized the oil-rich Abyei area. The South did not respond militarily, hoping to garner UN support. Not only was support withheld, but there was also no meaningful condemnation of Bashir's land grab. A senior UN official recently told me that the UN is controlled by the US and the US will not confront Bashir. Indeed, three UN experts reportedly resigned rather than be coerced into suppressing evidence against Khartoum. Why? President Obama is said to be sensitive about appearing anti-Arab and anti-Muslim in the wake of Iraq and Afghanistan.

Southern citizens now believe their only option is to protect themselves militarily. Mr Cameron, spare a thought for Kubra, that woman you met in the refugee camp in 2006. She entrusted you with her story and you promised to be her voice. Surely you owe it to her to find yours.

Tess Finch-Lees is a specialist in leadership, ethics and human rights