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.”