December 5, 1978

In an effort to prop up an unpopular pro-Soviet regime in Afghanistan, the Soviet Union signs a "friendship treaty" with the Afghan government agreeing to provide economic and military assistance. The treaty moved the Russians another step closer to their disastrous involvement in the Afghan civil war between the Soviet-supported communist government and the Muslim rebels, the Mujahideen, which officially began in 1979.
The Soviet Union always considered the bordering nation of Afghanistan of interest to its national security. Since the 1950s, the Soviet Union worked diligently to establish close relations with its neighbour by providing economic aid and military assistance. In the 1970s matters took a dramatic turn in Afghanistan, and in April 1978, members of the Afghan Communist Party overthrew and murdered President Sardar Mohammed Daoud. Nur Mohammed Taraki, head of the Communist Party, took over and immediately declared one-party rule in Afghanistan. The regime was extremely unpopular with many Afghans so the Soviets sought to bolster it with the December 1978 treaty. The treaty established a 20-year period of "friendship and co-operation" between the Soviet Union and Afghanistan. In addition to increased economic assistance, the Soviet Union promised continued co-operation in the military field. Soviet leader Leonid Brezhnev declared that the treaty marked a "qualitatively new character" of relations between the two nations.
The treaty, however, did not help Afghanistan. Taraki was overthrown and killed by members of the Afghan Communist Party who were dissatisfied with his rule in September 1979. In December, Soviet troops moved into Afghanistan and established a regime more amenable to Russian desires. Thus began what many pundits referred to as "Russia's Vietnam," as the Soviets poured endless amounts of money, weapons, and manpower into a seemingly endless civil war. Mikhail Gorbachev finally began the withdrawal of Russian troops nearly 10 years later.
Courtesy History.com