Justin Trudeau is a master of the moral high ground. "Canada is back," he proclaimed, promising that we will once again be a light unto the nations. Our foreign policy will reflect our values. Our environmental policy will square the circle of sustainability and economic growth.

Every new government promises to be more principled and high-minded than the previous one. They were sleazy and corrupt. We are ethical and pure. They did the expedient thing. We will do the right thing.

That's why the Saudi arms deal is so inconvenient. It shows that the government is swimming naked. It makes the new guys look just like the old guys – all too eager to do business with a regime that is not exactly a paragon of human rights.

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For a government that has made a fetish of reversing everything the previous government did (even if those things happened to be good), this looks awkward.

When the Conservatives authorized the Saudi arms deal back in 2014, high-minded liberals dumped all over it. "Principled foreign policy indeed," tweeted Gerry Butts sarcastically. Mr. Butts (an inveterate tweeter) is Mr. Trudeau's alter ego and his senior political adviser. Others were unhappy, too. "We've allowed an arms sale to trump human rights," said Roland Paris, a foreign policy expert who is now a senior adviser to Mr. Trudeau.

When the Liberals inherited the deal, they claimed that it was already done, that it was really no big deal at all (just "jeeps," as Mr. Trudeau described it during the election campaign), and that everybody else was doing it, too. That first part turned out not to be strictly true. As The Globe and Mail discovered, Foreign Affairs Minister Stéphane Dion signed off on most of the deal himself. Part of the rationale was that the "jeeps" might be useful in putting down the insurgency in Yemen, no doubt peacefully.

Well, you can't please all of the people all of the time. I wish Mr. Trudeau would come right out and say that the Saudis are officially our allies, and we have interests as well as ideals, and the world's a messy place, and not a thing we do or say will have the least influence on their conduct.

But that would hurt his brand as an idealist. Which brings me to the pipeline problem, where rhetoric also runs into reality.

Mr. Trudeau isn't against pipelines. But he isn't exactly for them, either. What he's for is doing the right thing. Unlike the ancien régime, he is determined to obtain what's called a social licence. This means that he will bring everyone to the table and listen to their views, and that all decisions will be based on facts and evidence. "Getting our resources to market … means doing it responsibly for communities, for indigenous peoples and for the environment," Mr. Trudeau says.

Who could disagree with that? The only problem is assuming that all the parties will agree on what the relevant facts and evidence are, and what "responsibly" means, and whether it is even moral to take the stuff out of the ground at all. The chance of a broad-based consensus on these points seems dim. Pipelines are no longer a business and regulatory issue. They are entirely a political issue, and for many people a moral issue. To them, the only good pipeline is one that's never built. They believe it's better to keep importing Saudi oil than to build a pipeline for our own oil to flow from west to east.

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Even Mr. Butts has sometimes sounded like a Leap supporter. (In case you missed it, the NDP's Leap Manifesto represents a leap back to a world of local, organic, pesticide-free farming powered by renewable energy sources, though presumably without the blight, the crop failures, the pitiable yields, the rural serfdom and the malnourished children that went with it.) As president of the World Wildlife Fund in Canada, Mr. Butts opposed pipelines to the Pacific. "The real alternative is not an alternative route, it's an alternative economy," he said then. "We don't think there ought to be a carbon-based energy industry by the middle of the century."

That is a stretch goal, to put it mildly. Mr. Trudeau's task will be to explain why more pipelines now can help us ditch fossil fuels and save the planet later. As with the Saudi arms deal, the moral arguments will loom large, even though they're pretty much beside the point. In fact, any pipelines we add to those that now crisscross the Earth will be utterly irrelevant to global warming (although not to our economy).

But Mr. Trudeau can't say that either. As he is doomed to discover, the truth is usually incompatible with moral posturing.