Option one: More intimidation 

But there are still options to exhaust before starting a war.

“Everyone is talking about invasion, but I can think of lots of things Putin could do short of invasion,” said Orysia Lutsevych, a Ukraine analyst at Chatham House. “And the West would be left wondering, again, what to do. Is that an invasion or isn’t it?”

Moves could include having Alexander Lukashenko, the dictator of Belarus, “invite” a Russian land force in to threaten Ukraine’s northern border, or openly moving “peacekeepers” into the Russian-controlled separatist republics in Donetsk and Luhansk.

Either step would simultaneously position Russia well for a future war, and place considerable additional pressure on Kyiv and Nato to do a deal before reaching that point, the Institute for the Study of War, a US think tank, argued in a study of Russia’s possible courses of action published last month. 

Option two: Punitive strikes

If intimidation fails, Mr Putin’s next move will likely involve his army’s greatest strength: long range artillery, rocket, cruise missile and air strikes that can pound enemies from afar with impunity.

Ukraine has no comparable force with which to retaliate and its air defences – mostly modernised Soviet-era missile systems – could down some Russian jets on bombing runs, but provide little in the way of anti-missile umbrella.

Nor does it have enough of them. Commanders would be forced to make painful choices about whether to defend frontline troops or critical infrastructure. Any launcher that unmasked itself to fire would immediately be vulnerable to destruction by Russian strikes.

Ukraine’s government is acutely aware of the problem, and has appealed to Nato for modern US Patriot systems – so far, to no avail.

But airpower alone has a poor record of rooting out entrenched forces, said Justin Bronk of the Royal United Services Institute.

If the air, missile and artillery campaign does not succeed in forcing concessions, it will be followed by ground forces.