If you're wondering what's happening in the Ukraine, these FAQs may help.
Is Russia going to stop at Crimea?
That's the big question right now. As shocking as Russia's move into Crimea was, occupying a small region where Russia already had a significant military and cultural presence is a far cry from pushing into the Ukrainian mainland, but Vladimir Putin does appear to have left that option open to himself.
The Russian parliament last week authorised the use of military force in Ukraine — not just Crimea.
Russia's justification for the Crimea operation was protecting the rights of ethnic Russians in the area, and this logic could theoretically be applied to eastern Ukraine, home to a significant population of Russian speakers.
After a conversation between Putin and Obama on Saturday (local time), a statement from the Russian president's office said that: "In the case of any further spread of violence to Eastern Ukraine and Crimea … Russia retains the right to protect its interests and the Russian-speaking population of those areas."
Pro-Russian protests were held in several cities in eastern Ukraine over the weekend, and violent clashes broke out as rival groups occupied a government building in Kharkiv, but Russian media reports of thousands of refugees headed for the Russian border appear to be inaccurate.
Ukraine is mobilising troops, but would likely find its 130,000-troop military badly outmatched if it comes to a firefight with the world's third-largest military by expenditure.
Why Is Crimea "strategically important"?
Access to the Black Sea has always been an important strategic priority for Russia, and it has had ships docked at Sevastopol since the late 18th century. The Black Sea Fleet has a storied military history. During the Crimean War, the city was besieged for 11 months by British, French and Turkish forces. During World War II, it held out for 250 days against the Nazis.
After years of dispute, Ukraine and Russia agreed in 1997 that Russia would be allowed to keep its fleet in Sevastopol until 2017. After the 2005 Orange Revolution, the staunchly anti-Russian President Viktor Yuschenko repeatedly threatened to expel the fleet, but his successor, Viktor Yanukovych, agreed in 2010 to extend Russia's lease on its naval base until 2042 in exchange for cheaper natural gas.
The fleet itself has decayed quite a bit over the years — it was briefly mobilised during the 2008 war with Georgia but didn't actually play a role.
Sevastopol isn't the only conceivable place where the fleet could be based. There have been discussions of converting the port of Novorossiysk on the Black Sea coast for the purpose, but that would take years, and given the history involved, a move out of Sevastopol would be seen as a major strategic retreat for Russia.
What can other countries do to punish Russia?
President Obama has promised "consequences" for Russia's actions, but options appear limited for America and its allies. The US, UK, France, Germany, Canada, Italy and Japan have all pledged to break off planning meetings for an upcoming G8 summit in Sochi planned for June, and Secretary of State John Kerry has suggested Russia may be kicked out of the grouping entirely, though the German government has expressed doubts about the idea.
The New York Times reports that the US has also "cancelled a trade mission to Moscow and a Russian trip to Washington to discuss energy while vowing to also scrap a naval-co-operation meeting with Russia".
Tougher measures that have been suggested include targeting the offshore assets of Russian elites and imposing travel or visa bans. Steps like these might be a tougher pill to swallow for European countries, which are still heavily dependent on Russia for oil and natural gas.
Others in Washington have suggested deploying Nato troops to countries bordering Russia and restarting shelved plans for a missile defence shield based in Poland.
On the other hand, the markets may be punishing Moscow more than other governments could, with Russia's stocks bonds and currency all taking a tumble Monday.
Who are the Tatars?
About 15 per cent of Crimea's population is made up of Tatars, a Muslim Turkic-speaking ethnic group who were the peninsula's original inhabitants. Deported en masse to Central Asia by Joseph Stalin, they only returned in large numbers to the region during the 1980s.
During the current unrest, they have generally been staunch supporters of the anti-Yanukovych protesters and have opposed Crimea's Russian nationalists, sometimes resulting in violent confrontations.