Ever since today's referendum in Crimea was announced two weeks ago the plebiscite has been mired in controversy, so much so that many outside Russia believe it is illegal and will only stoke up further problems.

At stake in the vote is the vexed question of whether the Black Sea peninsula should be allowed to become part of Russia or remain an independent state within Ukraine.

Tensions remained high yesterday ahead of the poll, with Ukraine's defence ministry scrambling aircraft and paratroops to confront Russian troops landing on a spit of land between Crimea and the mainland. Ukraine's foreign ministry demanded their withdrawal.

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The border guard service said talks between the two sides established that the Russian servicemen were "guarding against possible terrorist acts" on a gas pumping station, adding: "At this time, there is no threat of confrontation."

As expected, Russia yesterday vetoed a United Nations Security Council resolution that declared the referendum invalid.

"This is a sad and remarkable moment," Samantha Power, the US ambassador to the UN, said after the vote by the 15-member Security Council. Thirteen countries voted in favour, Russia against and China abstained. Russia and China are among the Security Council's five permanent, veto-holding members.

Polling stations will open for 12 hours today, with preliminary results expected within hours of polls closing tonight.

Crimea's electorate of 1.5 million, according to a format of the ballot paper issued last week, will choose one of two options - but both imply Russian control of the peninsula.

Many of the ethnic Russians who have a majority on the peninsula seem likely to back the first option on the ballot, union with Russia, if only for economic reasons.

A second option is independence, initially within Ukraine. The Crimean referendum seems to be a hasty affair which has been organised to take advantage of the recent turmoil in Ukraine and the collapse of the legitimate government led by president Viktor Yanukovich.

Normally referendums come about after lengthy consultation and the full agreement of all parties. But that is not the case in Crimea today.

On the one hand the Ukrainians, backed largely by the West, claim that there is not only no consensus for the move but that its outcome can never be legal. The argument is based on Article 73 of Ukraine's constitution which states that the country's borders can only be changed after a referendum of the entire Ukrainian people. As the Crimean administration has bypassed this step, to their way of thinking, the vote should not stand.

But Russia, too, believes that it has law on its side. To the Kremlin it is equally straightforward. Yanukovich sought sanctuary with them after he was toppled from power and as this was a criminal act on the part of the people of Ukraine the country's constitution is in abeyance. In the resulting vacuum Crimea is therefore free to explore the security of a connection with Russia and as some 60% of the people of Crimea are Russian speakers the odds are that today's vote will come down on the side of secession.

There is also an argument that when Ukraine took over responsibility for Crimea in 1954 it was a cynical piece of Soviet territorial engineering but historical details of that kind are now unimportant. What matters is that President Vladimir Putin will get what he wants and that Crimea will be quickly and smoothly subsumed into the Russian Federation. Yes, there is a slight of hand at work. Contrary to Moscow's propaganda, Crimea's Russian speakers have never been persecuted and, but for the ousting of Yanukovich, the question of swallowing up Crimea would probably never have arisen. It is also true that judging from the brutal behaviour of Russian forces in Chechnya, Putin has no particular love for local aspirations. In Syria, he has argued that the legality of the government of president Bashar al-Assad must be paramount.

But this is different. Crimea is not just a piece of historical territory where Imperial Russia once fought hard to hold off the intervention of the British, French and Ottoman empires during the Crimean War. Then as now it is home to Russia's Black Sea fleet which gives Russia a hold over the Mediterranean and the ability to project its naval power further afield. Without that secure anchorage Moscow's strategic options would be severely limited.

Besides, also at stake for Putin is a potent mix of pride and national security. In the aftermath of the end of the Cold War he has watched the slow but gradual encroachment of Russia's western borders as former Warsaw Pact allies such as Poland and the Baltic states threw in their lot with the Nato alliance. When it looked as if Ukraine was going the same way after the demise of Yanukovich it was essential to act to prevent the River Dnieper becoming a new frontier.

It remains true that the West is still beholden to Russia as a supplier of natural resources, especially Germany which imports up to one-third of its gas supplies from Russia. However, the West, especially the US, is threatening to impose sanctions but the harsh reality is that there is little that they can do about it. With last-ditch talks between US and Russian officials reaching no conclusion in London on Friday, everything hangs on which way the people of Crimea vote today.