One of the inconveniences for the government in its rush to war in Iraq is that it will complicate the process of getting rid of one of the government’s worst underperformers, Defence Minister David Johnston.

Johnston isn’t exactly alone in underperforming. The list of disappointments runs from Treasurer Joe Hockey down, and arguably only Finance Minister Mathias Cormann, Foreign Minister Julie Bishop and Sussan Ley — who is in the outer ministry — have performed strongly. He may not even be the worst performer, given Attorney-General George Brandis’ series of stumbles and the way Hockey has managed to relegate himself to the Last Chance Saloon after those “poor people don’t drive cars” comments. But Johnston is the most expendable, and his demise has long been rumoured.

The reasons why were on display last night in a key interview in which he struggled to give a clear answer to Leigh Sales on a question the government has been struggling with — whether we’re at war, as the Americans claim, or merely engaged in a “humanitarian operation”. That Johnston concluded that interview by first having to be corrected by Sales about al-Qaeda’s pre-9/11 capacity to launch major strikes on the West, then appearing to claim Islamic State had a capacity to launch attacks on the West similar to 9/11 — something explicitly rejected by US intelligence and security officials — demonstrated yet again why he is a particularly weak link.

Problem is, whether we’re at war or engaged in a humanitarian operation, removing the defence minister during the course of it won’t be a good look, and is potentially disruptive. There could be a positive: instead of having to use Immigration Minister Scott Morrison — who has handled Immigration effectively except for the fact asylum seekers tend to die in his custody — as the government’s articulate security tough guy, the minister supposedly in charge of such things could play that role. Indeed, Morrison’s name has been one of those bandied about for Defence.

But complicating that switch is that Defence is a cursed portfolio, from which few ministers make it out alive. Johnston’s immediate predecessor, Stephen Smith, left politics after Kevin Rudd returned. Smith’s predecessor, John Faulkner, took it on as his last ministerial position before moving to the backbench. Faulkner’s predecessor, Joel Fitzgibbon, crashed and burned over conflicts of interest, though he made a belated comeback in the Last Days of Rudd. Fitzgibbon’s predecessor, Brendan Nelson, lived to become opposition leader for five minutes, although he got into trouble for admitting that the 2003 iteration of the Iraq War had been about oil. Robert Hill lasted four years but then quit politics; Peter “children overboard” Reith lasted less than a year before leaving politics; it was John Moore’s and Ian McLachlan’s final gigs as well. If you go to Defence HQ at Russell Hill, history says it’s unlikely you’ll ever go anywhere else except out of politics, whether you’re good, bad or indifferent.

Dumping Johnston for Morrison or someone else (Hockey? It could work — stop wasting Turnbull in Communications and move him to Treasury, where he could no longer pretend to be somehow aloof from an inept government … I jest …) on the other hand might also signal that the government is serious about its commitment to the conflict it has waded into, that it isn’t merely a national security distraction but a challenge requiring the Coalition’s best skills. To put our defence forces in harm’s way, to place Australian citizens at great risk of terrorism because of it, to return to a region where we were part of one of modern history’s greatest mistakes, demands the best of a government, not a bloke basically in cabinet to recognise Western Australia’s outsized contribution to the Coalition’s ranks.