What was one of the major goals of the soviet union when it invaded afghanistan?

To rescue a friendly a dependent  socialist regime from external attacks.

Explanation:

Many professionals have been working since 1950 in Afghanistan. The number of professionals and projects increased after the socialist came to the power,

The soviet invasion was at 1979 which the numbers of troops reached around thousands. these took the economic and politic control. The antecedent of this is referred to the fall of the PDPA regime.  

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