The rise of cheap drones, whether reusable or expended as missiles, is playing havoc with the way armed forces have been equipped and trained.

Adequate answers are not obvious. Weapons and tactics that no longer work are easier to see than what will work.

So we can have some sympathy for Australian officers and officials who are trying to see the way forward - if in fact any are trying to see the way forward, instead of just hugging what has been familiar to them for decades.

Here's the problem: it's now possible to build a small, precisely guided drone, including a one-way drone, at a tiny fraction of the cost of a traditional aeroplane, helicopter or missile.

The price is also far less than the usual cost of shooting down an aircraft or missile, so defending against cheap drones will break a defence budget.

For the price of a frigate, the larger ones can be bought in the tens of thousands, the smaller ones in the millions.

I described this in an article in The Canberra Times this month. Now let's look at how Australia's army and navy are affected.

So far, the army looks like it's just carrying on as if the world hasn't changed.

Much of its re-equipment program is based on a century-old concept of military operations, best called "heavy combined arms", that the drones have overthrown.

It involves using a mix of big and expensive machines that fight the enemy in cooperation with each other and with infantry.

The army has tried for years to fully equip its three combat brigades with all the paraphernalia for that, wanting new tanks, infantry fighting vehicles, attack helicopters and so on.

The cost has been tens of billions.

This year's Defence Strategic Review cut the ambition to one brigade, which is apparently intended mainly to land on and seize nearby islands hosting Chinese forces.

Yet experience in Ukraine shows that the brigade's vehicles will be horribly vulnerable to little drones.

Beginning with the British invention of the tank in World War I, heavy combined arms evolved as a means of seizing well-defended territory.

But in Ukraine there is almost no seizing of territory these days by either side, largely because of thousands of cheap drones swarming over the battle area.

Setting up artillery or moving big vehicles in the open for an attack has become highly dangerous.

As for the Boeing Apache choppers we've ordered, their functions can be largely taken over by drones. Japan made that judgement even before Russia attacked Ukraine.

Last year this column pointed to experience in Ukraine showing our army's big vehicles would be vulnerable to attack by such drones as the Turkish Bayraktar TB2, costing US$1.5-2 million each, about one-twentieth as much as their victims.

That now seems quaint. If only the problem were so small.

Now we see that a US$2000 quadcopter drone, like the one you can buy for aerial videos, can be adapted to take out a tank or any other army vehicle.

The threat has indeed gone far beyond imperilling expensive machines: infantry can now be economically targeted with little drones that are, in effect, precision guided missiles.

So it's worse than 1917: soldiers are not reasonably safe even if they stay in their trenches.

What is the army telling Defence Minister Richard Marles about all this?

We don't know, but it can hardly pretend to have solutions that Ukraine and Russia have not thought of.

Heavy combined arms looks dead.

Then there's the navy. The government this month rejected a US request to send a warship to help defend merchant ships in the Red Sea against attacks by Houthi militants in Yemen using one-way drones - which in fact are propeller-driven cruise missiles.

The suspicion is that the navy could not easily make one of its 11 destroyers and frigates available for the task, but consider how futile the job appears to be, anyway.

The Houthis' missiles, supplied by Iran, cost perhaps tens of thousands of dollars each.

To shoot down anything, warship crews usually use multi-million-dollar interceptor missiles.

So far there is hardly any such thing as a naval counter-drone weapon, meaning one that's really cheap to use.

Any that are devised will operate only at a range that would be uncomfortably short for the commander of a multi-billion-dollar destroyer or frigate.

We can paint a picture much worse than what's happening in the Red Sea.

For US$10 million, about the price of 10 simple jet-propelled anti-ship missiles, like those that have worried navies for decades, an army could buy maybe 200 little propeller missiles.

They might fly towards an enemy warship 1000-2000 kilometres offshore.

Forget for a moment the utterly uneconomical cost of defence.

Even a big US destroyer just wouldn't have enough interceptors on board to down such a swarm.

The crew would have to hope that short-range guns and jamming would save the day. Fingers crossed.

Again, what's our navy telling the minister about all this?

It's probably looking at fitting counter-drone weapons to its ships.

If so, the minister needs to ask whether those guns or lasers would work fast enough to defeat a big swarm, and, if so, why shouldn't the swarm be even bigger?

Even more than most of us, armed forces resist change.

They like to keep doing what they've trained to do for decades.

The rise of the cheap drone makes that impossible.