ACCORDING to a treaty signed in 1994 by the UK, America and Russia, Britain is legally bound to defend the territorial integrity of Ukraine.
The Budapest Memorandum centred around unrest in Crimea, which was under Russian control until 1954, and formed part of a nuclear disarmament deal that saw Ukraine destroy all of its Soviet-era weapons.
In return, Russia promised to respect the new nation and not to threaten the territorial, political or economic independence of the state.
The pact is legally binding in international law, but relies mainly on goodwill between the nations involved. Russia is bound to similar rules by the UN Charter and other international treaties. If the rules are broken there will be consequences for Russia, but the United Nations and Nato are yet to agree on a course of action and military intervention looks unlikely.
The gradual weakening of the UN, infighting in Europe and economic problems in America have all contributed to fears that Russia could look to strengthen its influence in the region.
A lack of intervention in Syria over the last three years and in Georgia in 2008 have been interpreted as strong indications that the west has no appetite for getting involved in conflict.
In common with Ukraine, Russian intervention in Georgia was predicated on the country’s growing relationship with the EU. But Ukraine is a different story and there is significant pressure on Putin to de-escalate military action and withdraw troops from Crimea.
Both President Obama and David Cameron have warned Putin that “the world is watching,” but one country whose influence could turn events dramatically is China.
If it chose to back its neighbour Russia, it could become more difficult for Europe and America to resolve the conflict between Putin and his Ukrainian counterparts.
TIMELINE: ESCALATION OF TENSION
1954 - russia transfers crimea to ukrainian power
Russia annexed Crimea in 1783. In 1954 it transferred power to Ukraine but some ethnic Russians, 58 per cent of Crimea’s population, think this was a mistake.
1991 - soviet union collapses but russia keeps fleet in crimea
Muslim Crimean Tatars, once the majority in Ukraine, move back to Crimea after the Soviet Union collapse in 1991. This causes tension in the region. Russia positions the Black Sea Fleet on the coast.
1994 - treaty signed to protect sovereignty of ukraine
The Budapest Memorandum, signed by America, the UK, Russia and Ukraine is a promise to respect the territorial integrity of Ukraine and not to attempt to take back territory, including Crimea. In return Ukraine disposed of nuclear weapons.
2008 - Russia intervenes in georgia conflict
Russia, resentful of Georgia’s close ties with the West, sent troops into the country after it tried to reintegrate breakaway regions including South Ossetia.
2010 - Yanukovich elected and russian base lease extended
Yanukovich wins power despite a strong split between east and west. The Black Sea Fleet lease is extended amid opposition.