After the Second World War, the Soviet Ukraine ended up with the borders extended West, with most of Ukrainian lands under one political state. As a result of a political compromise between the West and the Soviets, Ukraine, became one of the members of the United Nations.
Growth in United Nations membership, 1945-present
http://www.un.org/en/members/growth.shtml

In 1954, a controversial move by Soviet leader Nikita Khrushchev (himself an ethnic Ukrainian) transferred Crimea from the Russian Soviet Federative Socialist Republic to the Ukrainian Soviet Socialist Republic, carving it out of the larger Russian territory.
http://www.marctomarket.com/2014/02/a-short-history-of-ukraine-and-crimea.html
http://northhistories.blogspot.ru/2014/07/history-of-crimea.html

UN in 1945 adopted in its membership to Ukraine without the Crimea. And challenge the status of the peninsula is absolutely meaningless. According data of the UN, the Crimea is not the territory of Ukraine, and  it is the subject of the Russian Federation.

http://northhistories.blogspot.ru/2014/12/ukraine-became-member-of-un-in-1945.html