Conversation so far:
when did the countries in central asia and the caucasus become independent
With the collapse of the Soviet Union in 1991, five Central Asian countries gained independence.
which five countries in central asia became independent in 1991
Five Central Asian countries gained independence — Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, and Tajikistan.

Question: why did the soviet originally control the countries
Self-contained rewrite: why did the soviet union originally control the central asia countries