The contest for eastern Ukraine may not be over, but when Ukraine's president, Petro Poroshenko, described the recapture of Sloviansk at the weekend as of "huge symbolic importance", he was not exaggerating. Other towns and cities remain in rebel hands, including the million-strong conurbation of Donetsk, but Sloviansk was the rebels' military headquarters. The hoisting of Ukraine's flag over the city hall marks a decisive advance for the government in the government in Kiev.

Which should raise a question: where are the Russians? If President Vladimir Putin was so intent on re-establishing Moscow's influence over Ukraine, if he was so determined to preserve Russia's fraternal ties with these fellow Slavs, if his ultimate objective was the reconstitution of empire, then why has he not rushed to the aid of those fighting, and dying, in Donetsk and Sloviansk?

Why have we heard nothing from Nato about Russian troops threateningly close to Ukraine's eastern border? Why no satellite pictures on our news bulletins of "clearly" Russian tanks rolling into east Ukrainian towns? Why no warnings recently from Washington or London about the dire consequences, should Moscow follow its annexation of Crimea by occupying eastern Ukraine?

Could this just be because the Russian aggression forecast so confidently in much of the western world is not actually happening? And if not, why not? There is an obvious answer to this, and a less obvious one. The obvious answer is that western sanctions against named Russian individuals, combined with market jitters about doing business with Russia, have had their effect. "We" have successfully stood up to a bully and faced him down; force has been met with a sufficient threat of force. Old-style containment, give or take the small detail of Crimea, has done its job. Russia has done the sums and seen sense.

But there could be another explanation for Russia's non-intervention in eastern Ukraine, which is that the widely accepted view of Putin's aggressive intentions was actually wrong. This other narrative fits the facts of the past five months just as well as Putin's presumed desire to act on his post-Soviet nostalgia.

As recently as September 2013, Putin was insisting that he had no problem with Ukraine's status as an independent, sovereign country, or even with its eventual membership of the EU. Then in February, Russia appeared to accept the agreement mediated by EU foreign ministers in Kiev, according to which the power of Ukraine's then president, Viktor Yanukovych, would be curtailed and elections brought forward.

That deal, however, broke down – for reasons hotly disputed by all sides. The western consensus is that Putin seized his chance and grabbed Crimea as the start of a lunge for all Ukraine, and even Transnistria and the Baltic states, too. But there is another possible explanation: that the combination of Yanukovych's sudden flight, the prospect of chaos spreading across Ukraine, and fears that the west could exploit the situation to fast-track Ukraine into the western bloc caused a degree of panic in Moscow that dictated what happened next.

Fear is a far stronger motivator for action than nostalgia. And Moscow's greatest fear at that point would have been the loss of its naval base and single warm-water port in Crimea. In the disorder that followed Yanukovych's overthrow, the Ukrainian parliament had shocked Russian-speakers by trying to downgrade the status of their language. Why should the cancellation of Russia's 25-year lease on Sevastopol not be next?

Shortly after Russia's annexation of Crimea, President Obama said that it reflected weakness and not strength. That was soon forgotten in the flurry of assertions that Russia had readied 80,000 troops to invade eastern Ukraine, some of them transferred from Crimea. But if weakness and fear, rather than strength and expansionism, lay behind Putin's actions, then the prime purpose of those troops was not offensive, but defensive. The intention, then, was not to seize a chunk of Ukraine or even – as a more sophisticated argument suggests – to make it ungovernable, but to bolster Russia's own security and prevent lawlessness crossing the border.

Russia's prickly responses are often explained by a centuries-old fear of encirclement. Through the cold war and since, that fear has been projected on to Nato – still seen in Moscow – however unrealistic it may seem to its cash-strapped members – as mighty, malevolent and terrifyingly efficient. If fear, rather than opportunism or strength, drove Putin's grab for Crimea, no amount of sanctions or belligerent talk from the west will improve things. They risk making Russia's insecurities – and its unpredictability – worse.