The invasion of Afghanistan was the Soviet Union's most expensive and direct involvement in a Third World conflict and many reasons have been suggested for it.
Collins, Joseph J.
"The Soviet Invasion of Afghanistan: Methods, Motives, and Ramifications,"
Naval War College Review: Vol. 33
, Article 6.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol33/iss6/6