Although Ukraine has a military force capable of making Russia think twice about invasion, it has a relatively light presence in the Crimea. Russia, by contrast, has for historical reasons a huge presence on the peninsula, with its Black Sea fleet based in Sevastopol.

"It is a nightmare for everyone," said Igor Sutyagin, a Russian military expert. "The entry of Russian troops would be a deep humiliation for Ukraine … It would be a second Chechnya."

Russia has an overall military force of about 845,000 troops against Ukraine's 130,000. Russia's military spending is also vastly greater than Ukraine's, $40.7bn last year compared with $1.4bn. But the Ukrainian forces are still formidable, better-trained, engaged over the last decade in international peacekeeping missions and established close contacts with western counterparts.

Brigadier Ben Barry, a specialist on land warfare at the International Institute for Strategic Studies, said: "If there was ever military confrontation, the question is how much the morale and fighting-power of the Ukrainian forces would be boosted by fighting for their country."

The small armed forces surrounding the two Crimea airports had no markings on their uniforms to identify them. Moscow denied responsibility but Kiev claimed the armed group at the Belbek airport, which is used by the Ukrainian air force and is close to Sevastopol, was made up of Russian marines.

Barry said that what was striking about the forces at the airport is they do not look like a newly formed militia. "This is not a ragtag force. When you see a new militia, they will have a jumble-sale look. This lot are uniformly dressed and equipped and seem competent and efficient, " he said.

Russia has put its combat planes on alert and has begun new training exercises, moves that prompted speculation of an impending invasion similar to the one into Georgia in 2008.

But all-out invasion of Ukraine appears unlikely at present given that even if Russia was to win, it would face years of costly and bloody insurrection. Taking over just Crimea appears, at least initially, to be less risky given that more than half the population is ethnic Russian. As a peninsula, Crimea would be theoretically easy to defend.

Ukraine has only a single coastal defence unit in the Crimea, about 3,500-strong, with artillery but no tanks.

But a Russian takeover of the Crimea could turn out to be disastrous in the long run. The Kremlin would be underestimating the impact of the sizeable population of Tartars who were forcibly deported from the Crimea by Stalin in 1944 and not allowed to return until the beginning of Perestroika in the 1980s.

Sutyagin, who is at the London-based Royal United Services Institute, said: "The Tartars are very anti-Russian. They will do anything not to be under the Russians. They will be determined to fight for Ukraine. It would be a second Chechnya. There are a lot of mountains in Crimea, just as in Chechnya."

Many of the soldiers fighting in the Ukrainian army are ethnic Russians but it would be a mistake to assume they might desert or turn on their officers rather than take on Russian forces. Sutyagin said loyalty to the idea of an independent Ukrainian state would top their ethnicity.

"The entry of Russian troops would be a deep humiliation for Ukraine. Ukrainians do not want to be occupied. It is a mistake by Russian politicians who think ethnic Russians are Russian," Sutyagin said.