Russia Ignoring Security Treaty For Ukraine

The Ukraine gave up it's nuclear weapons in 1994 after receiving assurances from the US, Russia, the UK and France to guarantee it's territorial integrity and security.  The security treaty was called the Budapest Memorandum on Security Assurances and was signed on December 4, 1994 by the Ukraine, Russia, USA and UK.   Later France agreed to sign it as well.

The treaty committed the signatories to abstain from the use or threat of force against Ukraine, to support Ukraine where an attempt is made to place pressure on it by economic coercion, and to bring any incident of aggression by a nuclear power before the UN Security Council.

Russia has obviously reneged on this treaty.  One could argue that the US and UK have theoretically honored the agreement by bringing the Russian invasion issue to the UN.  Not surprisingly Russia vetoed the UN draft of the Crimea referendum.  China abstained.

More about the Budapest Memorandum from Wikipedia:

The Budapest Memorandum on Security Assurances is a political agreement signed in Budapest, Hungary on 5 December 1994, providing security assurances by its signatories relating to Ukraine's accession to the Treaty on the Non-Proliferation of Nuclear Weapons. The Memorandum was originally signed by three nuclear-powers, the Russian Federation, the United States of America, and the United Kingdom and China.  France later gave individual statements of assurance as well.

 The memorandum included security assurances against threats or use of force against the territorial integrity or political independence of Ukraine as well as those of Belarus and Kazakhstan. As a result Ukraine gave up the world's third largest nuclear weapons stockpile between 1994 and 1996.