Kiev and Washington say Russia's current troop build-up near Ukraine is the biggest since 2014, when the Kremlin annexed Crimea and sent fighters and weapons into the Donbas region of eastern Ukraine to foment a war that has now killed 14,000 people.
The propaganda drumbeat is also growing louder in Russian state media, which say Moscow must be ready intervene in Donbas to protect its Russian-speaking residents from potential slaughter by “fascist” Ukrainian forces.
Many thousands of Ukrainian soldiers are themselves Russian speakers, and millions of those who voted for the country's president, Volodymyr Zelenskiy, are, like him, more comfortable speaking Russian than Ukrainian. The real aim of Russia's moves is not to rescue a Donbas region that the Kremlin's own war has ravaged but – as in 2014 – to warn Ukraine against Nato integration and remind the West that, in the final analysis, all its sanctions and stern lectures are no match for Moscow's armour in its own "backyard".
Vladimir Putin is also pushing back against new US president Joe Biden and diverting attention from his country's economic woes and the persecution of opposition leader Alexei Navalny and his allies, while seeking a ratings boost before autumn's parliamentary elections. Zelenskiy's own standing has been damaged by his failure to fulfil pledges to crush graft and end the war, and he recently adopted a tougher line against pro-Moscow politicians and media in Ukraine in an apparent bid to burnish his image; fears of major bloodshed in Donbas have pushed his country back up the international agenda, and made it a rallying point for the new US administration and its allies.
So Putin and Zelenskiy may see domestic political capital to be won and diplomatic points scored from the escalating tension in Donbas.
But sabre-rattling here is reckless: along a frontline that weaves through towns and villages, powerful shells fired from old and inaccurate artillery are just one miscalculation away from a block of flats or a factory, and a bloody spiral of unintended consequences.