The reason why Putin says that 'a new phase in the arms race is unfolding' is because this is how Russia sees US proposals to site anti-missile installations in poland and the czech republic. As Putin argues, Russia gave up forward positions at the end of the cold war - not that it had much choice; it had no means to continue stretching its military power that far - but the US and Nato have not done anything similar. It should also be noted that Nato has been somewhat reticent about the US plans to station its missile interceptors in europe (because it is divided over it).
The US has been pursuing agreements bilaterally with the countries concerned, outside the framework of Nato - which the European members of Nato should be concerned about. Anyway, so far as Russia is concerned, the institutional arrangements are immaterial. It sees what the US claims are purely defensive installations as offensive, and it will take quite a lot of persuading to convince Moscow otherwise.
This, rather than anything else, is the immediate rationale for Putin's position. I would absolutely dispute, however, that this is a sign that Russia will continue in its more assertive/aggressive stance under the likely next President, Dmitri Medvedev. So why did Putin make this the subject of his last speech to Russia's State Council?
1. it was his last speech in what is Russia's broadest official forum. It is his 'legacy' speech, just as the state of the union address was bush's. He has cultivated the image of a strong leader, even though this does not entirely correspond to reality. This is how he wants to be seen after he has left office, too. (and yes, he will leave office.)
2. there is a presidential election campaign underway in Russia. you may not think there is, and it is pretty token, because medvedev has Putin's support as heir apparent. But Putin knows that west-bashing is a vote-winner domestically. Therefore, it will be good for Medvedev's majority on 2 March.
3. putin may have wanted to counter the impression given by medvedev in a speech to business people last week that russia's assertive foreign policy stance had been counterproductive and was not necessarily in russia's interest's. i suspect that this is exactly what medvedev thinks, and that he will act accordingly as president, with a big overture to the west. but putin knows that to say so too loudly would be a turn-off for russian voters. So - as well as protecting his own legacy - he is trying to protect medvedev from himself in the run-up to the election. Medvedev needs a big majority if he is to inherit even some of Putin's authority.
