(London Daily Mail) A treaty signed in 1994 by the US and Britain could pull both countries into a war to protect Ukraine if President Putin's troops cross into the country.
Bill Clinton, John Major, Boris Yeltsin and Leonid Kuchma – the then-rulers of the USA, UK, Russia and Ukraine - agreed to the The Budapest Memorandum as part of the denuclearization of former Soviet republics after the dissolution of the Soviet Union
Technically it means that if Russia has invaded Ukraine then it would be difficult for the US and Britain to avoid going to war.
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Putin installed 150,000 troops along Ukraine's borders after the overthrow of Moscow ally Viktor Yanukovych by pro-European protesters.