MOSCOW, FEB. 15 -- The Soviet Union pulled its last troops out of Afghanistan today, ending an unpopular nine-year military intervention that severely strained East-West relations and cost the lives of nearly 15,000 Soviet soldiers. Lt. Gen. Boris Gromov, 45, the Soviet Union's last military commander in Afghanistan, crossed the steel bridge at the Soviet border post of Termez at 11:55 a.m. local time (1:55 a.m. Washington time). He was the last Soviet soldier to return home, meeting the deadline set by the April 1988 Geneva peace agreements. The general, who rode the last Soviet armored personnel carrier out of Afghanistan, was met on the bridge by his 14-year-old son, Maxim, who carried a bouquet of flowers. Together they walked the final few yards into Soviet territory. "I did not look back," Gromov later told reporters. The Soviet withdrawal from Afghanistan sets the stage for new diplomatic initiatives by the Kremlin, including improved relations with the United States and the Third World. Soviet President Mikhail Gorbachev travels to Beijing in May for the first Sino-Soviet summit in three decades, an event that was inconceivable as long as Soviet troops remained in Afghanistan. Hours after the Soviet troops crossed the border, the Kremlin reiterated a call made last month for an immediate cease-fire and an end to all arms shipments to Afghanistan. Moscow had earlier rejected such a mutual cutoff on the grounds that it would interfere with its obligations to the Kabul government. {Secretary of State James A. Baker III, speaking to reporters in Madrid, said the United States is "glad" that Soviet troops have left Afghanistan, ending "a tragic chapter in that nation's history." Baker said that at this point it is "appropriate for us to call upon the Soviet Union to assist in a reconstruction of Afghanistan." He did not elaborate, Washington Post staff writer Don Oberdorfer reported.} The Soviet Communist Party newspaper Pravda today came as close as Moscow has ever come to acknowledging publicly that the decision to invade Afghanistan, made by Soviet leader Leonid Brezhnev and a few close associates, was a mistake. A front-page article said that it was "possible to doubt . . . the evaluation of the military threat" posed by events in Afghanistan in December 1979. "It is possible to say that such vitally important decisions about the use of troops cannot be decided by a small circle of people, without the sanction of the parliament of the country," Pravda said. The paper insisted, however, that the Soviet Union never had "expansionist" intentions toward Afghanistan. The withdrawal of the last two Soviet battalions from Afghanistan leaves the Soviet-backed government of President Najibullah in tenuous control of the cities. Moslem rebels, who have been squabbling among themselves over the country's political future, control much of the mountainous countryside. Soviet officials said yesterday that about 30,000 mujaheddin rebels were massed in the snow-covered hills around Kabul, the Afghan capital. But western diplomats dismissed this figure as an exaggeration. United Nations observers were on hand to watch the last Soviet column of about 50 armored personnel carriers and 400 men cross the "Friendship Bridge" between the Afghan town of Khairaton and Termez. Over the past nine months, the Soviet Union has pulled out about 105,000 soldiers from Afghanistan. "We have fulfilled our internationalist duty to the end . . . but our grief for those who will not return will never be stilled," Gromov told a rally in Termez that was attended by about 5,000 local residents. A desolate outpost on the Amu Darya River, which marks the Soviet-Afghan border, Termez served as military headquarters for Marshal Sergei Sokolov, who commanded the invading troops in December 1979. Western intelligence officials believe that it was from Termez that the Soviet-backed Afghan leader Babrak Karmal announced the overthrow of his predecessor, Hafizullah Amin. In addition to almost 15,000 Soviet soldiers and officers who died during the war, nearly 37,000 were wounded, according to new figures published today in the weekly journal Literaturnaya Gazeta. On May 15 last year, when the Soviet withdrawal began, the Defense Ministry said that 13,310 soldiers had been killed and 35,478 wounded. Western sources estimate that more than 1 million Afghans have died during the war, which has also driven 5 million Afghans into refugee camps in neighboring Pakistan and Iran. Many Soviet commentators expect that the domestic controversy over the decision to invade Afghanistan is likely to heat up now that the last Soviet soldiers have returned home. Literaturnaya Gazeta, a publication widely read by intellectuals, today printed what is believed to be the first account of an atrocity by Soviet troops in Afghanistan to appear in the Moscow press. "War made {young Soviet conscripts} kill, burn and destroy," wrote Gennadi Bocharov, describing an incident in which a panicky Soviet Army unit killed a group of six Afghans, including two children, who were suspected of being supporters of the mujaheddin. "I don't need prisoners of war," a Soviet officer, identified as Capt. Rudikh, was quoted as saying as he ordered a heavy Army vehicle to crush a light car in which the Afghan civilians were being held. According to Literaturnaya Gazeta, which did not make clear when the incident took place, the captain was later pardoned immediately after being sentenced to six years' imprisonment. A soldier who carried out his orders is still serving a five-year prison sentence. In welcoming ceremonies for the last Soviet troops to cross the border, local politicians and Army commanders reiterated the standard Soviet explanation for the 1979 invasion. Gen. Nikolai Popov, commander of the Uzbekistan military region, said the troops were sent to Afghanistan "at the request of the legitimate government." About 250 Soviet diplomats are still believed to be in Kabul, where they are reported to be keeping a low profile behind the well-fortified gates of the Soviet Embassy. Gromov told a news conference in Kabul earlier this month that a "small group" of Soviet military advisers would also remain behind. Over the past few weeks, the Soviet media have prepared their readers for a flareup in the fighting in Afghanistan following the withdrawal of the Soviet troops and for the possible overthrow of the Najibullah regime. But Kremlin officials insist that any future Afghan government will have to take Soviet interests into account. "Whoever comes to power in Kabul will have to adopt at least a benignly neutral line toward the Soviet Union," said Radomir Bogdanov, deputy director of a Moscow political research center. "They will not be able to ignore facts such as our 2,000-kilometer {1,200-mile} common border and economic ties with us."