The focus of the crisis in Ukraine has shifted from Kiev to the Crimea region, as Ukraine accuses Russia of carrying out an armed invasion by sending naval forces to occupy Sevastopol airport.

The majority Russian-speaking Crimea region is of political and strategic significance to both Russia and Ukraine.

Russia's Black Sea Fleet has its historic base at Sevastopol. After Ukraine gained independence, a leasing agreement was drawn up to allow the fleet to continue operating from there.

In 2010, this lease was extended to 2042 in exchange for Russia supplying discounted natural gas.

The crisis began in November 2013 when President Yanukovych's cabinet announced that it was abandoning an agreement that would strengthen trade ties with the EU. The government later sought closer co-operation with Russia.

Anti-government protesters, who supported closer ties with the EU, called for the resignation of President Yanukovych and early elections.

There were protests across the country, but Kiev's Independence Square was at the heart of protests for three months.

Although peaceful for most of the time, bouts of violence injured hundreds and left more than 80 people dead.

As the violence escalated, the Ukrainian parliament voted to overthrow President Yanukovych and he fled over the border into Russia.

But the divisions within Ukraine go back much further than recent events. The country has been torn between East and West since the collapse of the Soviet Union in 1991 and this is reflected in a cultural and linguistic divide.

Russian is widely spoken in parts of the east and south. In some areas, including the Crimean peninsula, it is the main language.

In western regions - closer to Europe - Ukrainian is the main language and many of the people identify with Central Europe.

This division is to some extent reflected in voting patterns. The areas where a significant proportion of people speak Russian almost exactly match those that voted for Mr Yanukovych in 2010.

Ukraine has economic ties to both the European Union and Russia.

Russian gas pipelines to Europe pass through the country - a fact made abundantly clear in 2006 when Russia briefly cut supplies, sparking alarm in Western Europe.

The recent moves to reach a trade agreement with the EU again fuelled tensions with Moscow, which regards it as a step towards eventual EU membership.

Russia would prefer to halt Ukraine's closer integration with Europe in favour of boosting its own influence via a customs union.

The Russian military took up residence in Crimea more than 200 years ago, when Catherine the Great built a naval base at Sevastopol.

After the dissolution of the Soviet Union in 1991, Russia and Ukraine tussled repeatedly over dividing up the Black Sea Fleet based there. Today, Russia rents its Sevastopol base from Ukraine, where it has a presence better suited to bullying small neighbours than fighting a real war, according to Mark Galeotti, author of Russian Security and Paramilitary Forces Since 1991.

Galeotti, a professor at New York University who blogs about security issues, answered a series of questions about Russia's military.

A: As a war-fighting force, it's not particularly impressive. Its main vessel was basically built to fight other ships and so is only useful in fighting a naval war. It's got the Moskva, an aging guided missile cruiser; a large anti-submarine warfare cruiser - very dated; a destroyer and two frigates, which are more versatile; landing ships; and a diesel attack submarine. It's not a particularly powerful force. The Italian navy alone could easily destroy it.

A: It's moderately competent. It's not at the level of the American, or British, or German military, but it's better than in the 1990s. The [Russian] military is good at bullying small neighbours, but it would not be effective against NATO. It would not be able to defeat China.

A: The Black Sea Fleet had some value against Georgia because it was fighting a small navy. The Russian military could roll into Ukraine, but it would be up for a fight. The Ukrainians are rather more ready than the Georgians.

A: If Russia wanted to conquer the place, they could conquer the place. But what would it gain by claiming formal control over this region? It doesn't add up to me as being a takeover. It's a martial and heavy-handed political manoeuvre to make sure Kiev considers Russia's interests.

A: The main one is the fleet headquarters and naval infantry brigade headquarters at Sevastopol. There are four coastal missile regiments, four different bases. There are at least a dozen active bases on the Crimea. Some are just communication towers. Others are real bases. For example, there are several air bases.

A: The treaty between Ukraine and Russia limits the total size. Russia can't just add another ship to the fleet. They can't arbitrarily attach more units.

A: The 810 Naval Infantry Brigade has 2,500 marines. They're not elite, but they are better than average. They acquitted themselves well in Georgia and fighting pirates off Somalia. There are also some naval special forces. It's hard to be sure, but maybe 200 to 300. They may not be Green Berets, but they're pretty good. There's a large Black Sea Naval Air Force and ancillary groups - technical, security, administrative - you can put a gun in many of their hands, if need be. If you need people to block a road, they can do it.

A: There's nothing to stop them from moving troops around. You have slosh back and forth twice a year because of conscription. The sight of military personnel on the road is not unusual as they move between bases. And obviously they move back and forth to Russia.

A: They have a presence in Cuba. It's not really a base. It's a way station. And Tartus, in Syria. They have no other external bases.