ON display in the Tank Museum at Bovington is the 99-year-old machine that changed the way nations waged war.

Little Willie is the world’s oldest surviving tank – the first completed prototype for the machines that would take over from horses on the battlefield.

Even though motor vehicles were already replacing horses on Britain’s roads, the country entered the First World War reliant on cavalry.

David Fletcher, former historian at the Tank Museum, has said the cavalry was seen as ‘the epitome of style in warfare’. What’s more, a reactionary element in the upper echelons of the army could not bear change of any sort, he said.

But the slaughter of British troops and horses on the Western Front began to change attitudes.

Winston Churchill, First Lord of the Admiralty, created a ‘landships committee’ in 1915, to find a mechanised solution to the bloody stalemate of trench warfare.

Tractors with caterpillar tracks had already been developed and the agricultural firm William Foster & Co, of Lincoln, was hired to build a prototype armoured tractor.

The Foster’s Landship became known was Little Willie, apparently in an uncomp-limentary reference to the German Crown Prince. It could only do 2mph over rough ground and could not cross trenches, but its arrival was the beginning of a revolution in warfare.

Little Willie, with a six-cylinder Daimler engine, was up and running by the end of 1915, and took part in two demonstrations early the next year at Hatfield Park, Hertfordshire.

But a new design was already under construction – and it was Big Willie, or Mother, that became the prototype for Britain’s tanks in the Great War.

Little Willie was used for driver training before being moved back to Lincoln and ending up as part of the Tank Mus-eum’s first collection.

Big Willie was considered ready for action by 1916, when it was used at the First Battle of the Somme. These first tanks, known as the Mark I, had plenty of drawbacks of their own. The guns were in ‘sponsons’ hanging from the sides, instead of in a turret. The engine sat in the middle of the crew compartment, making conditions almost unbearable for the eight men around it.

But they could handle the rough territory of the western front, crossing a 5ft trench and reach a 4ft 6ins parapet.

They also had the distinctive rhomboid shape which became the famous outline of the tank.

The slaughter at Verdun and Somme in 1916 made the army keen to get as many tanks into action as possible.

On September 15, 36 tanks made an attack at the Somme. There should have been 50, but the remaining 14 had broken down or become stuck in the churned-up ground.

France was also developing a tank, but Germany lagged behind. It developed its own A7V, but only got as far as making 20, concentrating in the meanwhile on resisting the Allied vehicles.

A lighter, faster British tank, the Whippet, was in service by early 1918.

Tanks played an important part in a counter-attack against the Germans at Villers-Bretonneux in April that year, during which they engaged in combat for the first time with the few tanks Germany had. The machines were also important in the success of the Allied attack at Le Hamel and in front of Amiens that July and August, but the Tank Corps suffered heavy enough losses to neutralise it until late September. Even then, there were still too few to make a decisive difference at the end of the war.

It may not have represented the difference between defeat and victory, but the tank was here to stay, and it would be vital to future wars.

At the Tank Museum, an exhibition, From War Horse to Horse Power, tells the story of how the tank took over from the cavalry.

TV presenter and historian Dan Snow, who opened the exhibition earlier this year, said the tank “rewrote the rule book in war”.

He said: “There’s an extraordinary story of British engineering genius.