Armed men, believed to be Russian servicemen, take cover behind an armoured vehicle as they attempt to take over a military airbase in the Crimean town of Belbek near Sevastopol March 22, 2014. Shamil Zhumatov/Reuters

The career of Sergei Yeliseyev helps to explain why Ukraine's armed forces gave up Crimea almost without a fight - and why NATO now says it is alert to Russian attempts to undermine military loyalty in its eastern European members.

His rise to become number two in the Ukrainian navy long before Russia seized Crimea illustrates the divided loyalties that some personnel in countries that once belonged to the Soviet Union might still face.

Yeliseyev's roots were in Russia but he ended up serving Ukraine, a different ex-Soviet republic, only to defect when put to the test. NATO military planners now believe Moscow regards people with similarly ambiguous personal links as potentially valuable, should a new confrontation break out with the West.

In 2014, Yeliseyev was first deputy commander of the Ukrainian fleet, then largely based in Crimea, when Russian soldiers in unmarked uniforms took control of Kiev's ships and military bases on the peninsula.

Instead of resisting, Yeliseyev quit and subsequently got a new job: deputy chief of Russia's Baltic Fleet.

Yeliseyev, now aged 55, did not respond to Reuters questions sent to him via the Russian defense ministry.

In Kiev, however, there is no doubt where his loyalties lay. "When he took an oath to Ukraine, these were empty words for him. He has always been pro-Russian," said Ihor Voronchenko, now commander of the Ukrainian navy, who once served with Yeliseyev.

In fact, the Russian soldiers were pushing at an open door in late February 2014 - Yeliseyev was just one of many to defect and almost all Ukrainian forces in Crimea failed to resist.

Russia annexed Crimea the following month, prompting a major row with the West which deepened over Moscow's role in a rebellion in eastern Ukraine that lasts to this day.

Vice Admiral Sergei Yeliseyev, First Deputy Commander of the Ukrainian fleet, attends joint maritime exercises with Russian Navy forces in the Crimean port of Sevastopol, Ukraine on June 22, 2013. Stringer/Reuters

At the time, Moscow and its allies in Crimea exploited weaknesses within Kiev's military to undermine its ability to put up a fight, according to interviews conducted by Reuters with about a dozen people on both sides of the conflict.

The Russian defense ministry did not respond to questions on their accounts of the events in 2014 submitted by Reuters.

One NATO commander told Reuters that, in a re-run of the tactics it deployed in Crimea, Russian intelligence was trying to recruit ethnic Russians serving in the militaries of countries on its borders.

Speaking on condition of anonymity, the commander said the alliance was particularly sensitive to the risk in countries with high concentrations of ethnic Russians, notably the Baltic states.

NATO had to guard against this, said the commander, though the risk should not be overstated because having Russian roots did not necessarily mean that a person's loyalty is to Moscow.

Officials in the Baltic states, former Soviet republics which unlike Ukraine are NATO members, play down the danger.