When Soviet troops poured into Kabul a year ago today, most observers thought the Afghan nationalist resistance would collapse in the face of a modern army.

Instead, according to Western diplomats and neutral observers who have traveled recently through that rugged land, the resistance is flourishing.

The Red Army has 85,000 crack troops in Afghanistan, and the puppet government of Babrak Karmal in place. Yet those troops now have less control over the major cities and roads than they did a year ago.

While it is hard to accurately gauge the progress of the fighting in Afghanistan since the country is off limits to Western journalists, information reaching Washington and New Delhi (a primary listening post for events in Afghanistan) from a variety of sources indicates that government forces can travel on key highways only under heavy military escort, while rebels move openly through major cities, including the capital of Kabul.

"The Russians have discovered that the Afghans are not Czechs," said one diplomat, referring to the way Soviet forces were able to crush the 1968 reformist movement in Prague.

Very few people believed last year that it was possible for the rebels to hold out against the Soviets for more than six months, said one close Washington observer of events in Afghanistan since the Dec. 27, 1979, invasion there.

Those who did were considered hopelessly romantic and accused of placing too much faith in the traditional Afghan hatred of invaders, which gave the people of Afghanistan the will to force the stronger British Army into bloody retreat from Kabul in the mid-1800s. "But the British didn't have any tanks," said one diplomat.

Tanks or not, the Afghans have proved to be tenacious and ferocious fighters. Although they are far from united -- there are reports that the rebels have broken off battles among themselves to take on the Soviet force -- the rebel bands appear to have near-total support within the country.

"The Soviets have failed to achieve their minimum objectives either politically or in the field," said a State Department official. As evidence, he cited open battles between feuding factions of the ruling People's Democratic Party of Afghanistan, massive desertions from the Afghan Army (now down to about 30,000 men from a pre-invasion strength of about 70,000), and the increasing use of all Soviet military forces to try to crush rebel pockets of resistance.

"After a year, the only symbol of government authority in Afghanistan is the power of the Soviet Army," said the State Department official. But, he added, Moscow will need to double or triple its force in Afghanistan before it can pacify the resistance.

Other observers, looking back to the U.S. experience in Vietnam -- where half a million American troops failed to hold the country -- believe the Soviets will need to keep pouring forces into Afghanistan to gain control of it.

There are no indications, however, that the Soviets are preparing for any new massive increase of troop strength in Afghanistan, said the State Department official.

Some observers believe the Soviet need to deploy troops around Poland will inhibit troop increases in Afghanistan in the near future. But others think the Soviets are following a policy of trying to wear down the resistance without really conquering it.

Despite their inability to crush the rebels, the Soviets have gotten good value from their year of fighting in Afghanistan. The Red Army, trained and equipped to fight on the plains of either Europe or Mongolia, is getting its first combat experience since World War II ended 35 years ago. Its generals are being forced to develop new tactics to battle a homegrown insurgency on mountainous terrain.

According to diplomats, the Soviets are shuttling crack troop units in and out of Afghanistan to give wide segments of their Army combat experience. Some specialists, such as reconnaissance pilots, are brought in for four- to six-week periods so they can gain training under wartime conditions.

The rebels, meanwhile, appear to be hanging on only by sheer will, massive popular support, and a seeming ability to thrive under the harshest conditions. With their families parked as refugees in Pakistan, they are surviving the winter in caves eating flat bread and dried fruits and walking the mountain passes in open sandals.

Despite Soviet and Afghan government charges, diplomatic support from most of the noncommunist world has not been translated into active military aid, and the rebels fight with weapons that they have either captured from the Soviets or gotten from deserting Afghan soldiers. The more unwilling Afghans the Babrak government conscripts into the Army, the more weapons get turned over to the rebels, said one diplomat in New Delhi.