“The Ukraine” is incorrect both grammatically and politically, says Oksana Kyzyma of the Embassy of Ukraine in London. “Ukraine is both the conventional short and long name of the country,” she says. “This name is stated in the Ukrainian Declaration of Independence and Constitution.” The use of the article relates to the time before independence in 1991, when Ukraine was a republic of the Soviet Union known as the Ukrainian Soviet Socialist Republic, she says. Since then, it should be merely Ukraine.

See: https://www.bbc.com/news/magazine-18233844