When the Soviet Union broke up in 1991, Ukraine became an independent country.

The Russian President said last year he does not believe Ukraine should really be considered a country in its own right - more an artificial creation of the Soviet Union at a time when it was assumed Russia and Ukraine would remain in alignment.

A separate Ukraine, he believes, is a myth.

The Soviet transfer of Crimea to Ukraine in the 1950s when it was still a member - containing a strategically important Black Sea naval base - was also regarded by Vladimir Putin as a mistake - one which he took steps to reverse in 2014 when Russia invaded and annexed the region.

As Russian troops mass on Ukraine's border and with major joint military exercise planned in the coming weeks, Ukraine is feeling encircled.

NATO has openly said it fears Russian troops are poised for invasion.

Russia says that it too feels encircled. The 1991 break-up of the USSR not only gave independence to its former republics, Eastern European and Baltic States that had been occupied by the Soviet Union began to look westwards to guarantee their economic and military security.

Verbal assurances from the US in 1991 that NATO would not expand eastwards were never nailed down in writing in any formal pact or treaty.

By 1997, an agreement between NATO and the Russian Federation stated that NATO had "no intention, no plan and no reason to deploy nuclear weapons on the territory of new members".

Russia was conditionally - and reluctantly - agreeing to the Western alliance's expansion.

At the time Russia was in the throes of trying to make the transition from being the dominant player and the world’s second superpower to a modern economy.

Its bargaining power with NATO was weak. Moscow’s difficulty presented an opportunity for states it had invaded and occupied.

The West is worried about Russia, what's Russia worried about?
Russian, Ukrainian officials meet in bid to ease tensions
Russian ambassador to meet fishermen over naval exercise concerns

Vladimir Putin asked whether Luhansk and Donetsk breaking away from Ukraine would not be doing Kyiv a favour

The Czech Republic, Hungary and Poland joined NATO by 1999. Bulgaria, Estonia, Latvia, Lithuania, Romania and Slovakia followed suit in 2004.

In 2000, in the middle of all this, Vladimir Putin came to power in Russia.

When NATO expressed support for membership application plans by Georgia and Ukraine in May 2008, Mr Putin described the move as a "dire threat to the security of [Russia]".

Within months Russia had invaded Georgia after a conflict between the Georgian army and pro-Russian separatists in the regions of Transnistria and Abkhazia.

In Ukraine, matters came to a head in 2014. Ukrainian President Viktor Yanukovych departed from plans to move towards EU membership and instead chose closer relations with Moscow.

He was ousted in popular protests, calling for closer relations with the West - particularly EU membership.

In response, Russia ramped up support to separatists in the southeastern Ukrainian regions of Donetsk and Luhansk - collectively known as the Donbas region.

Whether Russia would be prepared to invade Ukraine in support of separatists - as in Georgia in 2008 - is a question currently occupying minds in NATO and Kyiv.

Last July, in a speech President Putin mused whether Luhansk and Donetsk breaking away from Ukraine would not be doing Kyiv a favour.

"Kyiv simply does not need Donbas. Why? Because, firstly, the inhabitants of these regions will never accept the order that they have tried and are trying to impose by force, blockade and threats," he said.

The optimistic view in Kyiv is that there are insufficient troops and supplies for the Russian build-up to be a genuine precursor to invasion

Late last year, NATO became concerned that Russia was holding exercises with tens of thousands of troops on Ukraine's border both within Russia and neighbouring Belarus.

More exercises are planned for next month in Belarus.

Russian troops remain deployed along Ukraine's border. NATO warned they were poised for invasion. Moscow denies this.

The optimistic view in Kyiv is that there are insufficient troops and supplies for the build-up to be a genuine precursor to invasion.

NATO has warned than any incursion will have serious consequences, including unprecedented sanctions.

How these troops will ultimately be used will be decided for Vladimir Putin.

Within Ukraine it is hoped that it will be limited to intimidation and not invasion.