It goes back way further than that, actually.
The Soviet Union was the one who first started referring to it as "The" Ukraine, when speaking about it as a Soviet territory within the USSR.
"Ukraine" roughly translates to "borderland" and the Soviets referred to Ukraine as "the borderland", both as a geopolitical definition, as well as one that had possessive connotations. Calling it "The Borderland" suggested it was never it's own unique area with its own customs or people, but an extension of Soviet territory. Hence, in translation, it became "The Ukraine". The West has been referring to it with the "The" prefix since the 1950's at least.
Fun fact: Ukraine HATES it when people call it "The" Ukraine, because it suggests a relationship with its former conquerer that it assuredly does not like.