By mid-1971, relations between Nimeiry's government and the Sudanese Communist Party had collapsed. The SCP had supported the 1969 coup and initially participated in the new government. Now Nimeiry wanted to phase out communist influence. He ordered the deportation of SCP Secretary General Abd al-Khaliq Mahjub. When Mahjub returned illegally after months abroad, Nimeiry placed him under house arrest. In March 1971, Nimeiry announced that trade unions, a traditional communist stronghold, would be placed under government control. The RCC banned communist-affiliated student, women's, and professional organizations. Nimeiry announced plans for the Sudan Socialist Union, which would absorb all political parties - including the SCP. After the speech, the government arrested the SCP's central committee. For the communists still at large, the choice was to act or disappear.

The coup began in mid-afternoon on 19 July, when Khartoum was at its quietest. Major Hashem al-Atta - one of the original Free Officers, now secretly retained by the SCP - moved tanks around the government buildings with minimal resistance. He captured the Presidential Palace and took Nimeiry prisoner along with several dozen loyalists. Atta then declared himself, Babiker al-Nur, and Farouk Osman Hamdallah in charge, proclaiming a new Revolutionary Council. Their first act was to lift Nimeiry's ban on the Sudanese Communist Party. Speaking over the radio, Atta announced the new government would collaborate more closely with communist and socialist countries. Sudanese communists, he promised, would form part of a new coalition government.

Al-Nur and Hamdallah were abroad when the coup happened. They boarded a British Overseas Airways Corporation jetliner in London for Khartoum, expecting to arrive as heroes. They did not arrive. Muammar Gaddafi, who had come to power in Libya two years earlier and was at this point virulently anti-communist, ordered two Libyan fighter jets to intercept the BOAC aircraft. The civilian plane was forced down in Libya. Al-Nur and Hamdallah were taken off the aircraft, arrested, and eventually handed to Nimeiry's forces. It was an extraordinary intervention - the deliberate forced diversion of a commercial passenger flight by a third country - and one that shaped the coup's final outcome as much as anything happening inside Khartoum itself.

Three days after Atta seized the palace, loyalist army units stormed in, rescued Nimeiry, and arrested Atta and his confederates. The counter-coup was swift. The reprisals were not swift - they were sustained. Nimeiry, blaming the SCP for the coup attempt, ordered the arrest of hundreds of communists and dissident military officers. Some were executed. Many more were imprisoned. Leading SCP members who had publicly backed the coup were hanged after summary trials. The Sudanese Communist Party, one of the largest communist movements in Africa or the Arab world, was effectively shattered as a political force. Sudan would never again have a substantial left wing of the kind the SCP had represented. Nimeiry, having survived, consolidated his rule. A provisional constitution that August described Sudan as a 'socialist democracy' even as the president was about to be elected to a six-year term with the country's politics pared down to his own preferences.

From cruising altitude Khartoum shows its distinctive three-city geometry where the Blue and White Niles meet. The Presidential Palace sits on the east bank of the Blue Nile, near the riverfront that forms central Khartoum's civic heart. Radio Omdurman - from which so many Sudanese governments have been announced and deposed - broadcasts from the west-bank city of the same name. During the 1971 events, tanks moved through streets that are visible from altitude today as the same pattern of boulevards and souks, largely unchanged in broad outline. The heat that empties the streets during siesta still empties them, every afternoon. The river still runs green-brown toward Egypt.