Nat. that launched 52-Across (4)

I believe the answer is:

ussr

(Other definitions for ussr that I've seen before include "Former Soviet Union" , "Former association of communist republics" , "Country established in 1922 and dissolved in 1991, in short" , "Former soviet states" , "Former federation of Russial and other states" .)

I've seen this clue in The Wall Street Journal.

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