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.
Putin installed 150,000 troops along Ukraine's borders after the overthrow of Moscow ally Viktor Yanukovych by pro-European protesters.
On Friday, as pro-Russia gunmen patrolled Crimean streets in armored vehicles and took over airports there, President Obama delivered a blunt warnings to Moscow.
"We are now deeply concerned by reports of military movements taken by the Russian Federation inside of Ukraine," he told reporters at the White House.
"Any violation of Ukraine's sovereignty and territorial integrity would be deeply destabilizing," he said in a brief appearance. "The United States will stand with the international community in affirming that there will be costs for any military intervention in Ukraine."
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