Quietly, the Soviet Union has opened a second front in its war over Afghanistan. It stretches from Leningrad, in the Soviets' wintry north, through the remote southern republics of Turkmenistan and Uzbekistan, and into Afghanistan itself.
The ''troops'' manning it do not carry guns and grenades. They tote pens and pencils, wrenches and hammers. Instead of tanks, there are cranes, forklifts, and bulldozers.
Their task is not to search and destroy, but to build: specifically, to forge links - economic, communications, transport - between the Soviet Union and its embattled southern neighbor.
Coupled with increased support for development projects in Afghanistan, the campaign underlines Moscow's long-term goal of fostering a viable, pro-Soviet regime there.
The Soviets' hope is to win gradual backing for such a government from traditional tribal and religious figures in the countryside. Late this summer, the Moscow-backed regime in Kabul reportedly rolled back on an earlier land-reform program and eased limits on acreage for friendlier rural chieftains. But the strategy seems to have done little to undermine village support for the rebels battling Soviet troops in Afghanistan.
In the short run, the Soviets' ''second front'' has one obvious military objective: to help supply the estimated 85,000 Red Army troops on Afghan soil.
The Amu Darya River, which snakes down from the Aral Sea in the southern Soviet Union along the frontier with Afghanistan, is being equipped for heavier traffic.
Soviet personnel and equipment are helping expand one Afghan river port (Khairatan) and to build another (Shir Khan).
Engineers in Leningrad have come up with a new hovercraft designed to tote up to 40-ton cargoes along the shallow water route. Shipyards in the Turkmenistan port city of Chardzhou, also on the Amu Darya, will produce the boats.
The Soviets are also helping mount facilities to service truck traffic south from the expanded port at Khairatan to Kabul.
A provincial Soviet newspaper reported in November that work was under way on the ''seventh span'' of a road-and-rail bridge between the Soviet and Afghan banks of the Amu Darya. ''It is planned to open traffic on the new bridge in May 1982,'' the report said.
Moscow has supplied equipment and other assistance for a direct radio link between the Soviet and Afghan capitals - and for a communications hookup between Khairatan port and Mazar-i-Sharif, the major northern Afghan town on the road to Kabul.
The railway station at Towraghondi, on Afghanistan's northwest border with the Soviet Union, is being rebuilt with Soviet help.
And experts in Uzbekistan have drawn up plans for a power link between the southern Soviet Union and northern Afghanistan, according to an Uzbekistan radio report.
Beyond military applications, the Soviet moves should further encourage a widening trade relationship between the USSR and Afghanistan.
Soviet-Afghan trade is one presumed factor in a current Afghan trade surplus, despite the ongoing guerrilla war, reported by the Reuter news agency Dec. 2.
Yet the Afghans' most profitable export moves not along the Amu Darya, but through a pipeline. This is natural gas for the southern Soviet Union, a commodity particularly helpful to the Soviets since revolutionary Iran shut off its gas exports to the USSR.