Heavily-armed soldiers have been a common sight in Paris after France was hit by deadly attacks, but in neighbouring Germany, talk about troops patrolling at home for the first time since World War II has sparked an emotional debate.

For many, Defence Minister Ursula von der Leyen set alarm bells ringing when she ordered soldiers on standby as an 18-year-old went on a gun rampage in a shopping mall in Munich in July.

The troops did not end up hitting the streets, for the attacker committed suicide following the shooting, in which nine people were killed.

But after a string of attacks in the same week -- including two claimed by the Islamic State group -- German Chancellor Angela Merkel presented a nine-point plan to bolster security, including training the military to respond to major terror assaults.

"It is now time to carry out exercises on major terror situations... which can involve the armed forces under the leadership of police," said Merkel.

Von der Leyen did not miss her cue and her ministry promptly confirmed the army and police were preparing for joint exercises.

But the possibility that the army may patrol at home for the first time in seven decades set off an outcry in the country haunted by its Nazi past, exposing a deep rift within the coalition.

Under the Nazi regime, the murky lines between the military and police in part enabled the regime to persecute Jews, Roma, leftwingers, gays and other declared enemies of the state.

Germany's post-war constitution has since drawn clear lines between the country's internal and external security forces.

It prohibits the Bundeswehr, as the federal armed forces are known, from deploying domestically, save for a few exceptions.

These include helping humanitarian relief in cases of natural disasters or emergencies, or in the event of a threat against "the free democratic order of the federal state".

- 'Nobody tells me why' -

The debate over the domestic role of the army has resurfaced several times, most recently in the wake of the November 13 Paris attacks targeting a concert hall and cafes that left 130 dead.

Finance Minister Wolfgang Schaeuble, himself a former interior minister, said troops should be used to bolster police forces should Germany be struck by a major terror assault.

Yet there is deep scepticism among the population.

A poll by Die Zeit weekly found that 66 percent of the population did not think that deploying the army in Germany was a good idea.

Junior coalition partner the Social Democrats were quick to lead the criticism, with their party chief Sigmar Gabriel batting off suggestions of a deployment and arguing instead that police should be given more support.

The police force itself also did not appear to welcome the help, with the chairman of the police union GdP, Oliver Malchow, saying that "the armed forces are completely unable to offer the help that we need."

"We need investigators, we need policemen who are trained constitutionally," he said.

But proponents seized on the July attacks to argue that it was better to be prepared.

Klaus Bouillon, interior minister in the western state of Saarland, warned that it would be "paradoxical and absurd, if the police were strained to the limit in an exceptional emergency while the army had to watch helplessly from the sidelines."

Christian Moelling, an expert on security and defence at the German Marshall Fund think tank, said the issue invited political posturing, with the two sides reverting to entrenched positions.

"I can't tell you if I'm for or against it because nobody tells me why -- why should the armed forces be used?" he said.

"An honest debate is needed, that is, what is happening on the ground and how can armed forces support with their capabilities the police forces," stressed Moelling.

He added that Germany has been "very lucky for the past years."

"We know that it is a target of terrorism, and we have been able to prevent almost every attack so far".