Leading article: An arms treaty and other reasons for East-West hope

A nuclear weapons agreement to be signed in Prague; a commemoration of the Second World War massacre at Katyn; civil strife in Central Asia... The headlines could have been written 20 years ago. But the differences and similarities between then and now show how far the post-Cold War world has come – and, at once, how much further there still is to go.

Today's meeting between the US President, Barack Obama, and Russia's President, Dmitry Medvedev, to sign a replacement for the Start treaty is the most promising development for many years in what used to be called superpower relations. While an agreement cutting the number of nuclear warheads on either side has a distinctly retro feel, this is about the future, not the past.

Mr Obama's decision to broach arms cuts as a mechanism for "resetting" the languishing relationship with Russia appears to have paid off. Arms control is a subject in which both countries still have abundant expertise. It is an area where they have a history of productive negotiation and where the Russians feel they operate on equal terms.

By Cold War standards, the agrreed cuts, at 30 per cent of the present number of warheads, are impressive. It can be objected that numbers mean less than they did in nuclear politics; that the configuration of Europe – with most former Warsaw Pact nations now in the European Union and Nato – makes cuts more of a boost for security on paper than in practice. And experts are already quibbling over the small print.

But the symbolism is eloquent and sends the much broader message that cutting, not acquiring, nuclear weapons can be the way of the future, if political will suffices. Agreement improves the general mood between the US and Russia, opening prospects for talks and co-operation in other fields, such as investment. It also allows Washington and Moscow to take the moral high ground in future dealings with Iran and anywhere else that might contemplate the acquisition of a nuclear capability.

As significant, on a smaller scale, was the solemn ceremony held near the Russian city of Smolensk yesterday. Arguments about responsibility for the Katyn massacre, in which the cream of Poland's officer corps was murdered, poisoned Polish-Russian relations throughout the post-war period. It was only in 1990 that Mikhail Gorbachev finally admitted Soviet guilt, but there were always those who contested this in Russia, even after the USSR's collapse.

Yesterday's commemoration, attended by the Polish and Russian prime ministers, cannot compensate for Poland's loss, but it sets history straight at state level – a precondition for a genuine normalisation of relations and, it must be hoped, for defusing some of the tensions that still bedevil EU-Russia relations.

No such optimism can be derived from the eruption of violence in Kyrgyzstan. Sporadic unrest has plagued the country since the "tulip revolution" of 2005 that drove the perestroika-era leader from power. Yesterday, the interior minister was shot dead, and troops fired on demonstrators in the capital, Bishkek, killing more than a dozen and injuring several hundred.

Whether this is a day-long ruction or something more profound, this stand-off underlines the perilous nature of the transitions that yet await much of Central Asia. The two largest countries in the region, Uzbekistan and Kazakhstan, still have ageing Soviet-trained leaders in charge. So, even as we seek to build on the positive changes that presage a safer world, a wary eye needs to be kept on the turmoil that may already be brewing further south.