By April 1992, Mr Najibullah had already surprised the world by managing to survive more than two years after Soviet occupation troops had given up and gone home. While the Soviet Union fell apart, its former puppet kept going. The Western diplomats who pulled out of Kabul with the Russians, assuming the billions of dollars their governments had given to the mujahedin would ensure the quick collapse of the regime, were made to look foolish.
For Mr Najibullah was never just a stooge. Like many educated Third Worlders, he was attracted to Communism as the only answer to his country's corruption and backwardness. Although he rose through the ranks of the secret police, where he presided over the torture of thousands, he was intelligent, energetic and uncorrupt. After Moscow chose him to replace Babrak Karmal, the man they invaded Afghanistan to put in power in 1979, he skillfully dissociated the regime from hardline Marxism. He managed to keep the armed forces as well as Afghanistan's urban classes on his side.
As he sits in his compound, wondering what he could have done differently, one name Mr Najibullah must curse is Benon Sevan. Mr Sevan, a UN diplomat trying to bring about a negotiated peace in Afghanistan, persuaded "The Ox" to make what turned out to be a crucial mistake: he agreed to step down before a deal had been made with his enemies. That convinced the regime's most powerful military commander, General Abdul Rashid Dostam, that it was time to switch sides, setting off a rush of peace deals all round the country.
Within weeks, the mujahedin, who had never mustered the discipline or tactics to have any hope of taking Kabul, were able to walk into the capital. Mr Sevan tried to smuggle Mr Najibullah out as a member of his entourage, but he was recognised at Kabul airport and had to turn back. He took refuge with the UN, remaining under the organisation's protection even after all international staff were pulled out.
Mr Najibullah, who like most Afghans has only one name, is looked after by local UN staff. His wife and children are in India. Sources say he passes the time by reading and watching BBC World Service Television. They refuse to confirm or deny the many rumours about him, among them that he has a serious kidney ailment.
In the first few months after his disappearance, it seemed the only way Mr Najibullah would ever leave his compound would be to go to his execution. The longer he remains alive, however, and the more his successors tear Afghanistan apart, the more he must be permitting himself some hope. He has never lacked self-confidence, and is only 48; perhaps he dreams of something more than simply surviving.