Lord Cameron said the West had displayed the same “weakness” when it failed to act after Russia invaded Georgia in 2008.
Meanwhile, the “uncertainty of the response” to the annexation of Crimea in 2014 had seen Putin “coming back to cost us far more with his aggression in 2022”.
“I want us to show the strength displayed since 2022, as the West has helped Ukrainians liberate half the territory seized by Putin, all without the loss of any Nato service personnel,” he wrote.
The Foreign Secretary suggested the West was underestimating its power to stop Russia, arguing that “we have the resources, the economic might, the expertise”.
He said the Kremlin had failed to act on previous support for Ukraine, such as the provision of tanks and long-range missiles, despite threatening retaliatory action.
“We have proved that the dangers of escalation are illusory,” he said. “Each time Putin has rattled his saber about escalation – and each time it has been empty rhetoric.
“Our economic strength outweighs Russia’s by a factor of around 25 to one. They are having to turn to Pyongyang for help. All we need to do is make our strength pay.”