Speaking at the end of the Nato conference, the prime minister sought to bolster faith that the conflict is worth fighting – arguing a Russian victory would worsen the economic situation.
Asked if he is worried about “Ukraine fatigue”, amid a cost of living crisis at home, Mr Johnson replied: “The point I would make about the cost of a freedom is that, actually, it is always worth paying.
“Unless we get the right result in Ukraine, Putin will be in a position to commit further acts of aggression against other parts of the former Soviet Union more or less with impunity.
“That will drive further global uncertainty, further oil shocks, further panics and more economic distress for the whole world.”
The “lesson of the 20th century” was that only the defeat of rogue countries who try to change borders by force delivers “a long period of sustained prosperity”, Mr Johnson said.
Asked what his war aims are now in Ukraine, the prime minister said it is up to Kyiv to decide whether to seek a peace deal at some point, while insisting that is not on the table at the moment.
“Whether they want to cut a deal with Russia, to make some kind of peace, of course that’s a matter for them,” he said, in Madrid.
But he added: “There doesn’t seem to be anything to talk about. Putin isn’t even offering a deal.”
The prime minister vowed to “intensify military support” for Ukraine, telling reporters: “The right thing for us is to keep going on the course that Nato set out, no matter how difficult it may be”.
“We think they do indeed have it in their power to repel the Russians and get them back to the pre-February 24 position,” Mr Johnson said.
He announced a new target for defence spending to reach 2.5 per cent of GDP by the end of this decade – at a cost of up to £13bn – amid controversy that he is about to break a current pledge.
Quizzed about problems at home, Mr Johnson denied that his government was “complacent” about inflation which is tipped to top 11 per cent this year, partly fuelled by the war.
He said he would look at the “root cause”, naming housing and transport as problem areas which he claimed were helping to drive “unnecessary costs”.
“I in no way minimise the importance of this. This is a big chance for us now to do things differently,” the prime minister insisted.