From 1989 to 1990, the Najibullah government was partially successful in building up the Afghan defence forces. The Khad had established a local militia force which stood at an estimated 100,000 men. The 17th Division in Herat, which had begun the 1979 Herat uprising against PDPA-rule, stood at 3,400 regular troops and 14,000 tribal men. In 1988, the total number of security forces available to the government stood at 300,000. This trend did not continue, and by the summer of 1990, the Afghan government forces were on the defensive again. By the beginning of 1991, the government controlled only 10 percent of Afghanistan, the eleven-year Siege of Khost had ended in a mujahideen victory and the morale of the Afghan military finally collapsed. In the Soviet Union, Kryuchkov and Shevardnadze had both supported continuing aid to the Najibullah government, but Kryuchkov had been arrested following the failed 1991 Soviet coup détat attempt and Shevardnadze had resigned from his posts in the Soviet government in December 1990 - there were no longer any pro-Najibullah people in the Soviet leadership and the Soviet Union was in the middle of an economic and political crisis, which would lead directly to the dissolution of the Soviet Union on 26 December 1991. At the same time Boris Yeltsin became Russias new hope, and he had no wish to continue to aid Najibullahs government, which he considered a relic of the past. In the autumn of 1991, Najibullah wrote to Shevardnadze "I didnt want to be president, you talked me into it, insisted on it, and promised support. Now you are throwing me and the Republic of Afghanistan to its fate."

Based on the above article, answer a question. When did the Siege of Khost start?