Russia and Ukraine both declared that Moscow’s offensive in the Donbas region had begun this week – although, in reality, the air and artillery bombardment by Moscow’s forces over the past 48 hours is not yet the intensive attack signalled by the Kremlin when it abandoned its attempt to seize Kyiv.

The strategy is well known: Russia aims to mass its previously overextended forces in the east of the country, where it hopes to gain an advantage of two or perhaps three to one over the Ukrainian defenders, encircling them by attacking south from Izium and, once Mariupol falls, pushing up from there north.

But the key point, for now, is that Russia is far from marshalling all its considerable remaining forces for the fight to come. Pentagon officials said on Tuesday that the attacks so far were simply “a prelude to larger offensive operations” that will determine whether the fighting amounts to a relatively short or far longer war.

After two months of fighting, the crude estimate from the US is that Russia has 75% of its combat power still available and 78 of its battalion tactical groups in Ukraine. At full strength, that would amount to about 62,000, though the figure is likely to be lower, plus thousands more separatists, mercenaries and other irregular forces.

Several thousand Russian troops – 12 battalions, estimates the US – remain tied up in the final stages of the battle for Mariupol, trying to take control of the vast Azovstal steelworks in the city. Others are still being reconstituted after the defeat near Kyiv, however, so the buildup is expected to be gradual.