Anders Fogh Rasmussen Addresses NATO members.
NATO Secretary General Anders Fogh Rasmussen speaks during a news conference at the Alliance headquarters in Brussels August 29, 2014. Russian forces are engaged in direct military operations inside Ukraine in a blatant violation of Ukraine's sovereignty and territorial integrity, Rasmussen said on Friday. REUTERS/Yves Herman (BELGIUM - Tags: POLITICS MILITARY)

Prime Minister Arseniy Yatsenyuk told the Ukrainian parliament Friday that his government is seeking to join NATO, more than two decades after the country became independent as the USSR dissolved. The move has already received strong condemnation from Moscow, but this isn’t the first time a former Soviet republic has turned to NATO for protection.

"The government is entering a bill to Verkhovna Rada (parliament) about the cancellation of Ukraine's non-bloc status and resumption of Ukraine's course for NATO membership,” said Yatsenyuk on Friday, as the country fights off what it called an invasion by Russia and seeks a path to gain the protection of the U.S.-led alliance.

The moves comes after NATO and Ukraine said that Russian troops had entered Ukraine in the east of the country. The invasion would be a breach of the Budapest Memorandum, a pact signed in 1994 by the U.S., Russia and the U.K. affirming that no signatory nation would threaten the territorial integrity of Ukraine, in exchange for the delivery to Russia or the destruction of the nuclear weapons that Ukraine had been left in the wake of the breakup of the Soviet Union.

Since the formation in 1949 of the North Atlantic Treaty Organization, which originally had 12 member states, there have been six phases of expansion. The first was in 1952 when Greece and Turkey joined. Over the next 35 years only West Germany (phase two, 1955) and Spain (phase three, 1982) became members. However, it wasn’t until the 1990s that NATO truly began to bring in more European countries, as the Cold War ended and former adversaries signed the NATO treaty and became alliance members.

In March 1999 NATO made its fourth expansion, bringing in Poland, Hungary and the Czech Republic, all former Soviet Bloc countries -- not former Soviet republics, but former members of the Warsaw Pact, the military alliance that opposed NATO, i.e. Soviet satellites.

Five years later the three Baltic states, Estonia, Latvia and Lithuania, joined, to date the only former Soviet republics in NATO. During that same expansion phase Bulgaria, Romania and Slovakia entered, as well as Slovenia, a former Yugoslav republic and the only country outside of the Soviet Bloc to join since Spain in 1982.

The sixth and more recent phase in 2009 saw Albania and Croatia, also formerly Communist, join. This means that the only former Soviet or Soviet-aligned countries in Europe that are not now part NATO are Moldova, Belarus, Ukraine, Georgia, Armenia and Azerbaijan. Ukraine is by far the largest of these.

Ukraine had been negotiating with NATO as part of the so-called Membership Action Plan since 2008, but the election in 2010 of pro-Russian President Viktor Yanukovych brought an end to that process, with the new president saying that there was "no question of Ukraine joining NATO" and that while the issue might "emerge at some point ... we will not see it in the immediate future." Yanukovych was overthrown this winter.

Russian President Vladimir Putin has firmly opposed Ukraine's NATO membership, calling it a “red line” that Ukraine must not cross.