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A former Nato commander expressed concern over a conflagration beyond Ukraine as analysts said Russia could move quickly to attack Kiev after launching an invasion of Ukraine.
Gen Sir Richard Shirreff said that Britain could soon be at war with Russia because any incursion into Nato territory would bring Britain into direct involvement in the conflict.
“There is no surprise and there is profound sadness and a sense of appalling horror at what is about to unfold for the people of Ukraine,” he said.
Britain was among the countries preparing a response after weeks of western threats and an escalation of sanctions in recent days failed to deter a Russian invasion.
Gen Shirreff, a former deputy supreme commander of Nato, said Britain had to “assume the worst” about Russia’s intentions.
“The Russians don’t hang around. They’ll look to establish overwhelming force,” he told BBC Radio 4’s Today programme.
“There will be multiple attacks from different axes … a full-blown military offensive to occupy Ukraine.
"I think we have to assume that this is not Russia biting off a chunk of Ukraine – for example, establishing a land corridor in Crimea – but a full-blown military offensive to occupy Ukraine.”
Gen Shirreff said it is “entirely plausible” that Russian President Vladimir Putin could be aiming to revive the Soviet Union. He said that if Russia puts “one bootstep” into Nato territory, the entire alliance will be at war.
Asked whether Britain might be expected to take part directly in the current military confrontation, he said: “Absolutely there is a possibility that we as a nation could be at war with Russia, because if Russia puts one bootstep across Nato territory, we are all at war with Russia. Every single one, every single member of the Nato alliance.
“Article 5 [of the Nato alliance] says an attack on one is an attack on all, so we need to change our mindset fundamentally, and that is why I say our defence starts in the UK on the frontiers of Nato.”
Franz-Stefan Gady, a military expert at the International Institute for Strategic Studies, predicted that Russian forces would “try everything to surround and smash the Ukrainian forces in several rapid pincer movements”.
He said Russian troops were fighting under the shield of a nuclear deterrent after Mr Putin spoke ominously of “horrible consequences” for anyone who wages war with Russia.
Mr Putin described Russia as one of the most potent nuclear powers and boasted of its range of state-of-the-art weapons.
“Ukrainian forces will have to fight well-organised delaying battles as best they can, the hardest type of tactical ground operations,” Mr Gady said.
“Without practice, they can quickly lead to chaotic retreats and break their fighting spirit. Breaking that spirit is the main aim of the rapid Russian operations.”
Ukraine’s border guards said Russian forces were attacking the country on several fronts after entering from Belarus, Russia and annexed Crimea. The Ukrainian government said shelling was taking place across the country.
The Kremlin had earlier said rebel leaders in eastern Ukraine had asked Moscow for military help against Kiev, days after Russia recognised separatist governments there.
Alex Plitsas, an Iraq War veteran and former US security official, said there were enough Russian troops to encircle Ukrainian forces in the east.
Those Ukrainian forces would then be “cut off from resupply and defending Kiev” if Russia launches an assault on the capital, Mr Plitsas said.
Although western leaders promised diplomatic retaliation against Russia, they have said they will not wage war on Ukraine’s behalf because it is not a member of Nato.
Josh Manning, a former US intelligence analyst, told Australian television that he hoped Ukraine’s fighting spirit on its home territory would be greater than that of Russian forces drafted in from far-flung barracks.
“My hope is that they fight hard and they overcome what is going to be an onslaught of brutal proportions,” he said.
Russia is “going to come in with power and shock and awe that the world hasn’t seen in a long time”.