The company has now demanded the federal government deliver “final certainty” about the pipeline’s future by May 31. The prime minister immediately called emergency meetings with his cabinet and the premiers of British Columbia and Alberta, who have antipodean hopes for the outcome.

Mr. Trudeau now has only limited options: Forge ahead, preparing to use sustained police force against demonstrators and designating public funds to minimize Kinder Morgan’s risk. Or let the project expire and make good on the Paris commitments.

On the home front, Mr. Trudeau asserts that pressing forward serves the “national interest.” He has repeatedly declared the “pipeline will get built” by citing a clause in Canada’s Constitution the government could employ to override provinces for projects of nationwide importance. Yet Canadians largely wouldn’t benefit under this scheme because most oil revenues don’t flow to public coffers, rather, increasingly to foreign companies. Even the supposed Asian demand has been vastly exaggerated. Exporting raw bitumen also would ship away potential refinery jobs. And legal experts argue that so-called national interest doesn’t trump Aboriginal rights that are equally enshrined in the Constitution.

Big-footing British Columbia and its First Nations, even if constitutionally viable, would likely damage Mr. Trudeau politically. Is he willing to tarnish his image and Canada’s peaceful reputation by deploying the military? He’s now in a familiar standoff against some of the country’s most tenacious environmentalists, reared in the birthplace of Greenpeace. In the 1990s, the “War in the Woods” protests against the logging of old-growth trees in Clayoquot Sound prompted major changes to provincial forestry policy; it was ultimately named a Unesco Biosphere Reserve. Fifteen years later, pitting the army against Indigenous grandmothers and teenagers simply for protecting their waters and the Earth could jeopardize 18 of his party’s parliamentary seats.

Mr. Trudeau all the while is keeping up appearances abroad, like recently vowing to “redouble” climate efforts with President Emmanuel Macron of France. That smacks of duplicity when a day earlier he offered public money to backstop the pipeline. Should the government guarantee investment, the company could attempt construction with minimized risk, and if that fails, head home while casting Canada’s politicians as an inept scapegoat (and potentially setting up a multi-billion-dollar lawsuit against Canada under Nafta). But it’s a long shot for sureties so soon.

Salvaging the embattled pipeline with tax dollars would be foolish and wasteful. It won’t halt civil disobedience that could stretch construction for years, nor beat mounting legal challenges by seven First Nations, two conservation groups and the cities of Vancouver and Burnaby. The lawsuits themselves pose an existential threat, contending Aboriginal groups were not adequately consulted, ecosystems and drinking water could be polluted, and orca and other endangered species will be harmed. (Kinder Morgan has already acknowledged in disclosures to shareholders that its permits could be voided.)

Whether he likes it or not, Mr. Trudeau is left with but one viable option: discard the pipeline and focus on greener pastures. To create jobs, he should ramp up the environmentally sustainable infrastructure program he campaigned on and pivot to the accelerating renewable energy sector.

Mr. Trudeau speaks persuasively about Canada’s leadership on climate, but in reality he’s trying for the impossible: to convince every side that he can please them. A pipeline will either go in the ground, or it won’t. The prime minister must show he’s worthy of his job before the 2019 election and act decisively. Rather than choosing between the provinces, or the oil lobby and eco-conscious voters, he should think hard about what best embodies his “sunny ways” principles — justice, science and saving the planet.