Ukraine crisis: Why Russia could invade and what is next.

Up to 130,000 Russian troops, alongside tanks, fighter jets and long-range missile batteries have amassed on Ukraine’s eastern borderPrime Minister Boris Johnson has on Tuesday confirmed that the UK will contribute to Nato deployments if Russia invades.

Claiming Moscow has a "gun to Ukraine's head", the PM warned Putin that he would not hesitate to "toughen our national sanctions" following a virtual meeting on Monday night with other Western leaders including US President Joe Biden.

Here we break down how the crisis reached this point and what could be next, Russian troops totaling 130,000, alongside 1,200 tanks, fighter jets and long-range missile batteries have amassed on Ukraine’s eastern border. The troop build-up has sparked the biggest crisis in East-West ties since the Cold War.

Russia has put forward a list of security demands including a guarantee that Ukraine will never be allowed to join Nato and that alliance forces pull back in Eastern European countries that joined after 1997.How Russia could invade Ukraine

Putin may seize territory up to Dnieper RiverMoscow has accused the US of ratcheting up tensions after Washington put several thousand troops on alert for possible deployment to boost Nato's eastern borders.A series of talks in various European cities this month have failed to ease tensions, though US Secretary of State Antony Blinken and Russian Foreign Minister Sergei Lavrov agreed at a meeting in Geneva on Friday to keep talking.

here has been tension between Moscow and Kyiv since Ukraine declared itself independent of the Soviet Union in 1991.The situation escalated in 2014 when Russia annexed the Crimean peninsula in southern Ukraine and sent troops to support a separatist uprising in the east of the country.

Ongoing clashes in Ukraine's Donetsk and Luhansk regions have killed around 14,000 people and two million people have been displaced.

All attempts to reach a political settlement have failed so far.

A peace agreement signed in Minsk in 2015 stopped the worst of the fighting but has failed to deliver a settlement of the conflict because of disagreements between Kyiv and Moscow over how it should be implemented.

Mr Putin maintains that Ukraine is fundamentally part of Russian civilisation, both culturally and historically, and has questioned whether it is even a real country.

Mr Putin also sees Russian dominance of Ukraine as fundamental to Russian security. The wider context is that the crisis is a challenge to what he views as an unfair

agreement imposed on Russia at the end of the Cold War.