My understanding is that although the Soviet Union was extensively involved in ‘wars of national liberation’ in the Third World, actually putting Soviet boots on the ground was something to be avoided - presumably both to maintain “plausible deniability” of the Soviet Union’s involvement as well as to avoid sparking a war should they directly come in contact with American / NATO troops. My question is, did the USSR ever attempt to circumvent this by using troops from other Warsaw Pact nations? And more generally in that vein - to what extent were the Soviet client states militarily involved in the Third World, as opposed to the Soviet Union itself?
While you are correct that the Soviet Union supported many communist groups in wars of national liberation, only a small number of USSR personal were sent to support these groups. Even fewer advisors and technicians from the Warsaw Pact nations were sent to these Third World Bush wars. However, Fidel Castro was more than willing to provide "boots on the ground." Particularly in Africa. Starting in Angola in 1975, with 25,000 troops, by 1987 there were 65,000 Cuban troops and civilian advisors operating in 17 different African nations. The Soviet Union provided the weapons, but the Cuban government paid their salaries. By 1987, 11 % of Cuba's annual government budget was being spent to support these wars of national liberation, but by 1991, the collapse of the Soviet Union and serious economic problems inside Cuba itself, lead to a dramatic scaling back of Cuban efforts in Africa.
http://www.foreignaffairs.com/articles/42294/pamela-s-falk/cuba-in-africa
Angola was the focus of Cuba's efforts in Africa. They provided 25,000 troops and 5,000 to 10,000 civilian advisors during a complicated, three way civil war that lasted from 1975 to 1991 in Angola. Contrast this to the 1500 Soviet military advisors, who primarily trained the Angolan communists how to operate Soviet provided military equipment. There were also 3,000 East German personal helping out the Communist Angolan government, but the East German were primarily working in the Railroad, Telecommunications and Intelligence sectors of the Angolan government.