Watch our mini-documentary The Letters of World War One

On Sunday, June 25 1916, Arthur Seanor wrote a letter to his girlfriend and his mother.

"I am just on the eve of going into the greatest battle the world has ever known," he began.

Arthur was due into conflict the next morning and knew the words he was carefully putting down on paper would only be read in the event of his death.

He did not know it then, but the next day's fighting at the Somme would be delayed by bad weather, fate that would keep him alive in the trenches of France for one more week.

Then, on the morning of July 1 - Arthur's 28th birthday - he shook hands with his superior and went over the top.

He was gunned down by German soldiers within 200 yards on what became - and remains - the bloodiest day in the British Army's history.

Arthur Seanor died at the Somme on his 28th birthday. Credit: ITV News

Some 19,240 British troops were slaughtered on the first of 141 days of gruesome battle at the Somme.

Arthur's body was seen in the bog on the battlefield but never recovered.

More than a century on, his final letter to his "own darling mother" is treasured by his family on Merseyside.

Arthur Seanor's nephew Peter Massen said he remains moved by his uncle's final written words. Credit: ITV News

Among his concluding words, he wrote: "I've done my duty best and stuck to everything I had to. And I'm proud to die a soldier."

Now prized documents, the letters to and from the First World War frontline were the only means of contact with the British servicemen in action between 1914 and 1918.

A staggering two billion letters and 114 million parcels were handled during the conflict by the British Army postal service, which was forced to quickly adapt to the feverish demand for communication.

How the postal service coped with the demand

Women filled thousands of jobs at the London Home Depot during the conflict. Credit: PA

The coordinating London Home Depot soon became the largest wooden structure in the world as it expanded under the weight of 1,050 tonnes of mail per week.

The UK operation also became a huge employer of women, with 35,000 recruited to cover the roles of the men who had gone off to fight.

The British Army postal service's networks ran through British and French ports. Credit: ITV News

At its height, 12.5 million letters were transported to the Western Front by the mail service, which also ran all the written coordinations between the frontline units.

A vast network was established, with a coded system of addresses to deliver to locations that needed to be kept secret from the enemy.

A postwoman does the rounds to deliver the letters from the frontline. Credit: PA

The correspondence now forms an extraordinarily detailed diary of events from the conflict, brought to life by messages like the one pilot Bernard Curtis Rice sent back to his father in November 1915.

Thanking him for the "excellent supply of apples" that had just arrived, in return he giddily reported his experiences in the burgeoning era of combat planes to give his father "some idea of what it is like flying over the (enemy) lines".

But how could the servicemen describe the events in the air and on the ground without endangering themselves and their units?

How soldiers wrote home without giving away secrets

The information shared during the war was subject to huge censorship. Credit: ITV News

Beyond the extraordinary war-time distribution of letters was the enormous undertaking of censorship.

Each letter was subject to a strict checking process to prevent vital information - innocently shared - being intercepted by spies.

For frontline soldiers, that careful screening was the duty of their unit officers.

It meant men like Ralph McGuire, writing home to "his princess" from Greece in 1916, could only hint at the horrors he had witnessed after three journeys to the frontline.

Ralph McGuire's letter to his 'princess' from 'somewhere in Flanders'. Credit: Imperial War Museum

"You can have no idea what it is like," he wrote. "The night seems the worst.

"We do nothing on daylight, just stay in our dugouts."

Though he offered a brief description, he went on: "You will have to wait until I come home for experiences and news in general.

"These letters are of course censored and I don't want to say anything that I should not say."

Many men were also reluctant to share the full extent of their feelings under the eyes of their superiors.

Green envelopes ensured soldiers' letters would be read beyond their unit. Credit: ITV News

Green envelopes were the solution for those who wanted to limit the amount of censorship subjected to their letters.

It ensured their words would be sent to be read instead by an outside officer, unconnected to their unit.

A far quicker device was also introduced to avoid the need for censorship altogether.

The First World War 'status update'

The simplified Field Service Postcard bypassed censorship. Credit: Imperial War Museum

The Field Service Postcard became a kind of social media status update for its time, with soldiers able to choose between a few descriptions to describe their health (from "I am quite well" to "I am wounded"), confirm receipt of a letter or parcel, or report a breakdown in communication.

While the messages were impersonal, they were gratefully received by those desperate for news of their loved ones and remain among the fascinating elements of First World War communication passed down from generation to generation.

The lasting treasures, though, remain the longer letters detailing the human experience of every element of the conflict, right up until the final shot was fired before the German Empire surrendered.

Writing to celebrate the Armistice

Crowds gather to celebrate the end of the war in November 1918. Credit: PA

Read today, the hopeful words of soldiers like Brasher Clarke are loaded with poignancy.

Writing to his father on November 11, 1918, Armistice Day, imagining the celebrations back home, he said: "May history record that this was the war that ended war and God grant that such madness be never unleashed upon the world again.

"If we the Allies can build a new ideal world from the ruins of the old one it matters little what becomes of the Kaiser and his brood.

Brasher Clarke's letter, written on Armistice Day, is full of optimism. Credit: Imperial War Museum

"The sacrifice of so many of our bravest and best will not have been in vain. Let us have a real League of Nations."

Many men like Brasher, finally returning home, could perhaps not foresee the lasting personal damage the conflict would take on their generation.

Nor would they have believed that the lasting peace, so hard fought, would be so quickly undermined.

For all their efforts and all their sacrifices, Britons would be writing home describing the horrors of another world war within two decades.