The threat of invasion drew the British Army to Norfolk's beaches, fields and river crossings twice in the early 20th century.

And while those who manned the networks of trenches, gun batteries and road blocks have long since moved on, there is one battalion of sentinels that never left.

Brick or concrete pillboxes - small forts from where gun crews could fight off invaders - still pockmark Norfolk's north and eastern coastlines, ever-present reminders of a storm cloud which mercifully never burst.

And over the past two years, a Worstead man has been researching a lesser known defensive ribbon once known as the River Ant Stop Line.

Before the pandemic Steve Smith, 54, used to work as a battlefield guide, showing people around First and Second World War sites in France and Belgium. But changing times have meant his focus has shifted closer to home.

"These are things that we walk past or drive past every day but a lot of people don't recognise the significance of them," Mr Smith said.

"What you don't see now are the trenches, the barbed wire and the camouflage that was there. Norfolk effectively became a great big fortress.

"If you move past one line you come across another defensive position that would have given the Germans a headache. And it wasn't just the Home Guard defending these positions, the more important ones were defended by regulars or the territorials."

Although the threat of invasion in the Second World War and Hitler's plans for 'Operation Sealion' have become infamous, the fears over a German onslaught in the First World War are less widely known.

Norfolk's pillboxes date from both conflicts, and although there are more surviving from the Second World War, there are many still intact from the first.

Mr Smith said: "Most people would assume they are all from the Second World War, but I don't think they realise that in the First World War, the War Office had a fear the Germans might try to land on this coastline."

A map of the River Ant and North Walsham and Dilham Canal shows at least 10 First World War pillboxes along the waterways and even more than that from the following war.

Further inland were other defensive lines running north-west to south-east across Norfolk and Waveney, mostly following the course of rivers - the strongest being the General Headquarters (GHQ) Line, which was considered to be the last, best chance of defence.

Mr Smith has tried to visit as many defensive sites along the River Ant as he could to confirm they are still there - seeking permission from landowners for the structures on private property.

He said his research was "not yet complete" and there was more about the history of Norfolk's defences that he wanted to uncover.

Christopher Bird, from Kingston upon Thames, has been researching Norfolk's wartime defences for more than 40 years and has penned a book on the subject called Silent Sentinels.

He writes of the Great War threat: "In January 1916 the General Staff decided that the Germans were able to carry out more than mere nuisance raids, calculating that they might be able to land up to 160,000 men on the east coast, and that it could take the Royal Navy 24 to 28 hours to arrive on the spot. The General Staff considered such an attack unlikely, but that precautions should be taken."

But the defences did not always work according to plan. Mr Bird writes: "Most First World War pillboxes had thick steel doors, but this was very unusual on their Second World War descendants.

"Some..had skeletal doors, which would have been little use in combat, and were more likely designed rather to keep meddlesome children out."

But the doors seem to have been more trouble than they were worth.

"One evening two members of the Home Guard went to check on a pillbox on Horstead Common only to be locked in when the door jammed," he writes.

"Since no-one could hear their shouting, they were forced to spend the night in the pillbox. By the next morning, a Sunday, they were growing desperate, and seeing the villagers going to the nearby church, they fired their guns in their direction to attract their attention.

"Miraculously no-one rang the church bells, and the weary men were let out."

Mr Smith has written several books about Norfolk's military history, the most recent being The Norfolk Regiment on the Western Front 1914-1918.