Russia has been accused of ignoring international law by sending troops to Ukraine. US President Barack Obama says the move is in clear violation of Ukrainian sovereignty, and there will be costs for any military intervention.
Russia says it is to protect its citizens. Ukraine has responded by placing its army on full combat alert and has warned of war.
There have been months of protests in Ukraine after President Yanukovich refused to sign a trade deal with the European Union, opting instead for closer ties with Russia.
Now the strategic region of Crimea has been caught up in the crisis. The largely self-governed region is part of Ukraine but the majority of its population are ethnic Russians.
But what can diplomats do to defuse what is threatening to become to the biggest confrontation between the West and Russia since the Cold War?
Presenter: Folly Bah Thibault
Guests: Sergey Markov - spokesman for the Russian president, Vladimir Putin
Alisa Lockwood – security and political risk analyst, HIS Country Risk
Valentin Yakushik - a political science professor at the University of Kiev's Mohyla Academy, and an advisor to the Ukrainian president.