Justin Trudeau's opponents constantly try to brand him as unready for office, and the Liberal Leader keeps proving them wrong. Until lately, that is. Suddenly we've been hit with the temptation to say, Quiet, Justin. The adults are talking.

It's hard to imagine what Mr. Trudeau was trying to prove when he said the government should provide humanitarian aid and non-combat support to the coalition taking on the Islamic State – "rather than whip out our CF-18s and show them how big they are." Did he think his remark was funny? Hip? Accurate?

Between that comment, his reference in the Commons to the CF-18s as "a few aging warplanes" and his party's weakly reasoned argument against a combat role for Canada in Iraq and Syria, Mr. Trudeau has performed poorly on the biggest file to cross his desk since becoming party leader.

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The nub of his argument against a combat role is that "it is always easier to get into a war than to get out of one." Yes, it can be easier to get in than out, but he has offered no evidence that the deployment of six CF-18s, one refuelling plane and two surveillance aircraft will grow into something larger. He is not saying it will, and he's not saying it won't. He's just saying, darkly, it could.

Okay, but you still have to make a decision. The question as defined by the government's motion was, Should we or should we not join our friends and allies – the U.S., Saudi Arabia, France, the U.K., Germany, Bahrain, the United Arab Emirates, Australia, the Netherlands and others – that have committed their air forces to bombing IS positions? It's that question that Mr. Trudeau has avoided answering directly.

Thomas Mulcair, the Opposition Leader, wasn't any more compelling in his refusal to support the combat mission. "The tragedy in Iraq and Syria will not end with another Western-led invasion in that region," he said. It is a big stretch to qualify this as an invasion, and a significant number of the active participants are not Western countries but Middle Eastern ones.

At least Mr. Mulcair is being consistent with his role as Opposition Leader and with the core values of the NDP. Mr. Trudeau, on the other hand, leads a third party that, when in government, showed a willingness to take the risk of deploying troops. Key Liberals – Roméo Dallaire, Lloyd Axworthy and Bob Rae – support a combat mission. Mr. Trudeau, though, dismisses combat as macho posturing and Canada's military capabilities as "a few aging warplanes." If he were prime minister, is this what he would tell Canada's allies?