The answer is that in 2010 elite opinion somehow coalesced around the view that deficits, not high unemployment and weak growth, were the great problem facing policy makers. There was never any evidence for this view; after all, those low interest rates showed that markets weren’t at all worried about debt. But never mind — it was what all the important people were saying, and all that you read in much of the financial press. And few politicians were willing to challenge this orthodoxy.

Most notably, those who should have stood up for public spending suffered a striking failure of nerve. Britain’s Labour Party, in particular, essentially accepted Conservative claims that the nation was facing a fiscal crisis, and was reduced to arguing at the margin about what form austerity should take. Even President Obama temporarily began echoing Republican rhetoric about the need to tighten the government’s belt.

And having bought into deficit panic, center-left parties found themselves in an extremely weak position. Austerity rhetoric comes naturally to right-wing politicians, who are always arguing that we can’t afford to help the poor and unlucky (although somehow we’re able to afford tax cuts for the rich). Center-left politicians who endorse austerity, however, find themselves reduced to arguing that they won’t inflict quite as much pain. It’s a losing proposition, politically as well as economically.

Now come Justin Trudeau’s Liberals, who are finally willing to say what sensible economists (even at places like the International Monetary Fund) have been saying all along. And they weren’t punished politically — on the contrary, they won a stunning victory.

So will the Liberals put their platform into practice? They should. Interest rates remain incredibly low: Canada can borrow for 10 years at only 1½ percent, and its 30-year inflation-protected bonds yield less than 1 percent. Furthermore, Canada is probably facing an extended period of weak private demand, thanks to low oil prices and the likely deflation of a housing bubble.

Let’s hope, then, that Mr. Trudeau stays with the program. He has an opportunity to show the world what truly responsible fiscal policy looks like.