Question: When I think of Britain, I think of a dominant global empire. Was this not the case? Was Britain completely incompetent outside their navy? With respect to WWI.
Short Answer:
The 1914 British Army (975,000 men) was a quarter the size of France's or Germany, 17% of that of Russia. On top of that the Empire's army was needed elsewhere. This is reflected by it's anemic offering to meat grinder which wasthe continent for War I (1914-1915, BEF had 250,000 men). France had casualties of 250,000 in a single battle during 1914. In a different time training and professionalism might have mitigated the lack of manpower, However in WWI when the tried and true professional tactic of charging across the open field and attacking the enemy, met the machine gun; experience was as much a liability as an asset. It would not hide the dearth of numbers.
Answer:
Just prior to WWI, the British Empire was at the height of its power and global influence. It encompassed about a quarter of the world. It was the Worlds largest economy, the leading industrialized base, and of coarse the worlds largest Navy. And yet the British standing army, including reserves was small relative to France, Russia, and Germany. The British army just prior to WWI in 1914 including reserves consisted of 975,000 men. About a quarter of France and Germany's armies, and about 17% of that fielded by Russia. Worse still Britain could only bring about a quarter of their army to bare for the first two years of the war. Although Britain would ultimately out mobilize France during the war from 1914-1915 the British Expeditionary Force consisted of only 6 divisions or 250,000 men. Given France suffered 250,000 casualties (80,000 deaths) during 1 battle, first battle of the Marne (Sept 12, 1914) you can clearly see the problem.
Why was Britain so understaffed? Britain employed a small professional army to police it's empire. When WWI broke out it didn't negate the need for that police effort, remember Ireland war of Independence, Australia had an active Independent movement, Britain had fought two wars in South Africa in the last few decades, and South Africa's Boer population were culturally Dutch, German and pro German in WWI. India with a population greater than Britain could not be neglected either. Britain could not send their entire force to the Continent to fight WWI, many were already engaged in work important for the Empire.
Then what exposed BEE'sBEF's manpower issue which might have been mitigated by experience, training and professionalism was the unprecedented carnage of WWI. Military technology had developed(**) since the last great power war, but Military tactics to overcome such advances had not. Thus the nature of WWI exasperated the lack of manpower in a time when the war was placing an overwhelming priority on manpower.
(**) Not only had the machine gun come of age, but also because of the science had revolutionized the production of nitrogen necessary for the manufacture of munitions and explosives. No longer were industrialized nations limited by their access to guano (bat shit) to produce munitions, now such countries could produce unlimited nitrogen out of thin air. see Habar Process