James Frayne is Director of Public First and author of Meet the People, a guide to moving public opinion.

If the Mail on Sunday is right, Gavin Williamson has demanded £20 billion for defence from the Prime Minister over the next decade – or risk him bringing down the Government. It’s an extraordinary story, but are backroom power plays like this the only way to make the case for higher defence spending at a time when it sits so low on the public’s list of priorities? Can you ever mobilise public support for higher defence spending through more conventional campaigning? These questions need asking because the defence establishment has been actively making the case for continued investment for years now – apparently to little positive effect.

In truth, it’s hard to say that it has really thrown the kitchen sink at advocacy. It has tended to focus on three main areas. Firstly, it’s focused on jobs – the quantity and quality of them. While the argument that we need a strong defence industry base for national security reasons has been made occasionally, much more common have been the arguments that the sector provides high-skilled jobs in areas that would otherwise be left economically desolate. (Few people that work in Westminster will have failed to see BAE Systems’ massive marketing exercises in Westminster tube.)

Secondly, the wider establishment has focused on international development, and the need for Britain to have a strong military that can do good around the world in humanitarian and aid terms. This has often featured in recruitment ads and it was said to have been one of the reasons Britain ultimately went for an independent carrier force.  And, thirdly, it has focused on the need for Britain to retain influence as a global power. We need a strong military to sit at the top table.

You will notice that I haven’t picked out – other than in passing – national security and war-fighting. That’s not a mistake. For this has typically been a much less audible argument from the defence establishment. You surprisingly rarely hear them talking plainly about the need to protect the country against external threats. General Nick Carter made this case earlier this year in a high profile speech at RUSI, but it was memorable for how unusual it was.

Up to a point, this is understandable and makes sense. In all but the most blatant cases, it’s hard (and unwise) for secretaries of state and defence chiefs to start talking up threats from particular countries. And that’s certainly not the role of industry representatives. But in focusing on pretty much everything but the fundamentals of what the armed forces are for – and why they need adequate funding – the defence establishment has failed to capture the imagination of the public, and are now flailing, as the Prime Minister seemingly questions the need for continued high levels of spending.

The defence establishment ought to re-focus the debate on national security. That doesn’t mean telling everyone to prepare for the next war and apparently baiting any hostile country that will listen for one. Rather, it means some of the following: highlighting the need to give British soldiers the kit they need to stay safe and perform effectively in training and combat; talking about national security in the broader sense, so people don’t just think about tanks and aircraft but about things like combating terrorism; highlighting the positive and negative innovations that other countries are pursuing through their armed forces; and, finally, emphasising the need for Britain to play a leading role in NATO, which guarantees our safety and which demands we more than pay our way.

In other words, the defence establishment should stop talking about defence spending as if was all about apprenticeships and training, as important as these things are, and get people focused on public safety. Without engaging the public, they’ll be reliant on the bravado of sympathetic secretaries of state whose shelf-lives are invariably short.