When the clouds of World War I first gathered, Britain and its empire were less than fully ready to weather the storm.

But together they managed one of the biggest and fastest mobilisations of military power in history, which would have implications for the war and for the future of the empire itself.

In the summer of 1914, Britain had a professional army of 400,000, half of it garrisoned around its empire, as well as 300,000 reservists and territorial forces.

On August 4, when it entered the war, it had a little more than 150,000 operational troops to send immediately into battle in Europe — a quarter of the force deployed by Germany.

However, by the end of September a rush of patriotism saw over 750,000 more sign up on home soil and by the end of the year one million had enlisted.

Australia, Canada, the Indian subcontinent and other parts of an empire on which it was said the “sun never sets” also sprang into action.

“Our boys were not just Tommies — they were Tariqs and Tajinders too,” junior British foreign office minister Sayeeda Warsi said in a speech marking the war’s centenary.

Unlike other powers, Britain had not fought a major European land war for a century. Due to the patchwork of European treaties that saw London drawn into the war, the naval power had boosted its armed forces in the years before 1914 but its troop numbers were still dwarfed by its allies — and enemies.

The mobilisation fell to Lord Horatio Kitchener — the mustachioed secretary of state for war who featured on an iconic recruitment poster pointing at the viewer above the words “Your country needs YOU”.

“In August 1914 he was almost alone among the country’s principal soldiers and statesmen in predicting that the war would be a protracted and costly business,” wrote historian Peter Simkins in Kitchener’s Army.

Largely thanks to Kitchener’s efforts, 478,893 Britons enlisted between August 4 and September 12 — 301,000 in the two weeks after August 30 alone. On September 3, the busiest single day of the war for recruitment, 33,204 joined up, Simkins said.

There was a huge expansion of recruitment centres, while age limits were raised to 38 and the minimum height lowered to five feet, three inches (1.60 metres).

In total, 2,466,719 enlisted between August 1914 and December 1915.

Britain did not introduce conscription until 1916, after many of the volunteers had been mown down on the battlefields of northern France. By the end of the war in 1918, at its maximum strength, the British army had nearly four million in uniform.

But another key to the huge mobilisation lay with the colonies and dominions of Britain’s empire.

Modern-day India, Pakistan and Bangladesh sent 1,500,000 troops, nearly 500,000 came from Australia and New Zealand, the same number from Canada and 74,000 from South Africa, among other countries, according to British government figures.

Andrew Fisher, Australia’s soon-to-be prime minister, quickly pledged support to the British empire  “to the last man and the last shilling” — a move greeted with great public enthusiasm.

The 400,000 men from Australia alone represented 10 per cent of the population at the time. Of those, 61,000 died in what is generally accepted as a defining moment for the young nation.?AFP