The "war on terror" looks chillingly "war-like" when thousands of khaki-clad soldiers walk the streets of your homeland armed with military assault rifles.

Until this week, the Queen's Guard was about as visible as the British Army got in the cities and centres of the United Kingdom.

But in ordering "Operation Temperer" — a previously unused and secret plan to respond to critical security threats — Theresa May has marshalled a formidable show of force to her people and to those seeking to cause them harm.

If British eyes are taking a while to adjust to the sight of troops outside Parliament in Westminster and at 10 Downing Street, they may soon be subject to full visual military assault.

Commanders of Operation Temperer have the power to deploy soldiers at football matches, concerts and flower shows, and nuclear power plants — and in some cases the order has already been issued.

Why did they do it?

Although Tony Blair sent a few hundred troops and tanks to protect airports against a 2003 terror threat to bring down passenger planes, British prime ministers have taken extreme caution in using domestic martial powers in peacetime.

It's visually confronting, it can imply the state is rattled and — a crucial consideration for a British leader — it risks making the mainland look like Northern Ireland in the late 20th century.

Weighing these risks, Ms May called the troops out of barracks for one simple reason: she can.

While the Queen is the Commander-in-Chief of British Armed Forces, the effective power to order military operations rests with Ms May and her Cabinet — there is no Parliamentary vote and, unlike Australia, there are no states to negotiate with.

If British eyes are adjusting to the sight of troops at 10 Downing Street, they may soon be subject to full visual military assault. ( AP: Lauren Hurley )

Are ADF sentries coming to an event near you?

The Australian military is routinely "called out" for domestic operations, especially disaster relief, explosives disposals and even to help with searches.

But a call on the Army to aid a security operation is almost as rare in Australia as in the UK. By general consensus, the last time was in 1978 as part of the Commonwealth Heads of Government Regional Meeting, which was rocked by the Hilton Hotel Bombing.

Reasons for sparing the use of the power are mostly legal, but are built on a moral principle too. Australia's military was created to protect the country against "invasion" and no man or woman has ever enlisted to use lethal force against fellow Australians.

But a combination of terrorism, modern weaponry, vast geography and the varying strengths of state police forces are now prompting national security policy makers to ponder the question:

Could a modern military call-out be made quickly, decisively and legally at a time of great threat and stress?

From the Federal Government's perspective, the answer appears to be "no", which is why Attorney-General George Brandis is finishing a submission to Cabinet on the subject.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 4 minutes 2 seconds 4 m 2 s Lindt Cafe siege: Govt examines call-out laws for terrorist hostage situations ( Tom Iggulden ) Download 7.4 MB

"It is important that when there is a major terrorist episode of the kind we've seen in the United Kingdom, that all of the capabilities the country has that may be useful to deal with that terrorist event are at the disposal of the operational commander," he said.

The coronial inquiry into Sydney's Lindt Cafe siege found "the challenge global terrorism poses for state police forces calls into question the adequacy of existing arrangements for the transfer of responsibility for terrorist incidents to the ADF".

How to ask for military help

Australia has a national defence force because the founding fathers in the constitution decided "the Commonwealth shall protect every State against invasion".

If a state was experiencing what the constitution describes as "domestic violence", then the Commonwealth could provide protection for that too, but only if a state asks ("on the application of the Executive Government of the State", is how it's written).

Once a state, via its Governor, has decided that "domestic violence" (it's not defined, but could include riot, revolution, siege or hostage-taking) is taking place, a request must go to Canberra to be considered under the Defence Act and its various "instructions".

In the past, British prime ministers have taken extreme caution in using domestic martial powers. ( Reuters: Neil Hall )

Using the military to help restore law and order where a state felt its own police force was not up to the task would be considered as "Aid to the Civil Power" and an anxious Premier would have to wait for a Prime Minister, a Defence Minister or the Federal Cabinet to consider it and respond via the Governor-General.

The Federal Government is well advanced on a review of military "call out" arrangements.

The details are secret, but the Lindt siege Coroner wants it to work out whether "further guidance is required on the criteria to be used by states and territories in determining whether to apply for Commonwealth assistance".

'Who ya gonna call?'

Even if the requesting and handover arrangements from state police to ADF are streamlined, that still leaves practical questions about who has got the best skills, training and equipment to carry out tasks and who would command them.

At one end of the spectrum, posting general infantry soldiers to stand outside sporting events and concerts is relatively low risk (assuming they'd never have to fire a shot in anger).

At the upper end (involving heavily armed terrorists, bombs and mass hostage-taking), the scenario quickly gets more complicated.

If the event was in downtown Sydney and NSW Police felt it was beyond their capability, commandos in the ADF's Tactical Assault Group (East) could provide some capability — trained and backed by recent experience storming and clearing buildings in hostile situations in Afghanistan.

If the event was in Darwin or Launceston, their practical ability to offer a rapid response would be limited.

It's unlikely British-style "Operation Temperer" troop deployments would become the norm in Australia. ( Supplied: AD (file photo) )

How about a paramilitary?

National security officials involved in the federal review of military call-out arrangements have been examining the growth in European countries towards highly trained, heavily armed, hybrid forces which "slide" between military tasks and civilian counter-terrorism operations.

France, with a starkly different constitution, has its National Gendarmerie Intervention Group (GIGN) sitting within its armed forces structure, but its most notable counter-terrorism operations have been carried out domestically.

Germany's GSG 9 (Border Protection Group 9), on the other hand, is a police unit which has also run counter-terrorism operations on at least three continents.

Former Army chief Peter Leahy has cited both as examples of a rapid-response paramilitary Australia should consider raising and basing in all major population centres, but to be based on state and federal police forces — not as part of the ADF.

When will we know?

Senator Brandis has indicated he will finish his Cabinet submission for legal changes to call-out arrangements "in the near future".

Most national security bills these days clear Parliament with bipartisan support and minor amendment.

But even if the legal procedures are changed, it doesn't mean British-style "Operation Temperer" troop deployments would become the norm in Australia.

Given our history and constitution, if legions of Army soldiers ever take to the streets here, it would be proof positive that the nation is in security peril.