What were the reasons for the Soviet-Afghanistan war and Vietnam war?

The Soviet-Afghanistan War started in 1979 when the Soviet Union replaced the Islamic radical leader Hafizullah Amin with the Communist Babrak Karmal.  The Soviet Union sought to lessen the influence of Islamic extremism and potential Western influences near its borders.  This is key because the Soviet Union has long had problems with extremism in its own territories in Chechnya.  The ensuing war was a disaster for the Soviets as American-backed rebels fought the Soviet army to a standstill.  This would be the last major war of the Soviet regime.  

America assisted South Vietnam against Communist North Vietnam initially by using advisers.  After the Gulf of Tonkin incident, where the U.S.S. Maddox was attacked off the coast of North Vietnam, Lyndon Johnson ordered more troops sent to South Vietnam to assist in fighting both the North Vietnamese Army and the Vietcong.  America fought in the Vietnam War due to the domino theory, which stated that if one country fell to Communism, the other nations around it would soon follow.  America worried about its position in Asia and did not want Communist takeovers to happen in the Philippines and Japan.  It was important for American politicians not to look soft on Communism for political reasons back home, and also it was supposed to improve the morale of our European allies who felt threatened by Soviet encroachment.  The logic was that if America fought against Communism in the developing world, then it definitely would fight the Soviets if they decided to invade Europe.