What were the circumstances each great power faced in developing a military during World War I?

The great powers of World War I all faced unique challenges and circumstances when mobilizing their militaries during the conflict.
Germany
On August 2, 1914, Germany began officially mobilizing for war. At the time the country had a sizable standing army and pool of reserves, numbering at 4.5 million soldiers. However, active recruitment soon swelled those numbers considerably as young German men eagerly enlisted. From the start Germany expected the war to be brief, telling its soldiers that they would be home by Christmas. As a result, the country did not make serious adjustments to its economy to support its military for a prolonged conflict. This led to severe food shortages both for soldiers at the front and the populace back in Germany.
Austria-Hungary
Austria-Hungary had been developing strategies for an offensive long before the outbreak of hostilities. However, the empire's military was not prepared for the modern weapons of the era and suffered heavy losses early on. The commanders and war planners were still using tactics of the previous century, which resulted in high casualties in the face of machine guns, heavy artillery, and poison gas. Overall, the Austro-Hungarian military was woefully underdeveloped. It lacked a proper military budget, and its technology was antiquated compared to its opponents. Being a multi-ethnic fighting force, it also faced the challenge of coordinating operations between units in multiple languages.
Italy
Italy's army was vastly outnumbered by those of Germany and Austria-Hungary. The country was never able to compete in terms of numbers or leadership with its foes. It mostly fought along its northern border in the Austrian Alps. Losses were heavy, numbering in the hundreds of thousands. Before the war, Italy had been in a defensive alliance with Austria-Hungary and Germany. Therefore, the country had never developed much of a plan for fighting against these nations. There was little money for waging the war. The fight essentially destroyed the economy of the nation which was never able to adequately supply its army as a result.
Great Britain
The British army was relatively small compared to the other European powers that fought in World War I. This is because they never drafted soldiers. Rather they relied entirely on volunteers to form their fighting forces. The British military was relatively well equipped though. They had a booming economy prior to the war and were consequently able to keep their fighting forces well supplied with the latest in military technology. Great Britain also had the largest and most advanced navy of its day. However, the quick mobilization process meant that many officers had little or no actual combat experience, and coordination between various units was often unorganized.
France
France had been practicing conscription for years before the start of the war. Therefore, it had a large army, but not a professional one. France also had large colonies in Africa and Asia whose populations helped augment its forces. France relied heavily on fixed fortifications to keep the German advance at bay. This proved disastrous as the German army easily outflanked them by attacking by way of Belgium. During the war, the French readily adapted to using new technology, especially the machine gun, the armored tank, and long-range artillery.
Russia
Russia had the largest army at the start of the war with nearly six million troops ready for combat. That number was quickly doubled once hostilities began. Despite these large numbers the Russians were poorly equipped and technologically outmatched by the German army. Military and political leadership was greatly divided and unorganized as well. This led to massive losses. When the tsar was overthrown in 1917, Russia surrendered and left the war.
The United States
The United States' military contribution to the war was relatively small in terms of personnel. When it entered the war, the expeditionary army numbered just over 200,000. The country drafted an additional four million soldiers. However, a successful economy meant that American soldiers were well equipped with ample supplies and the latest in wartime technology. Moving soldiers and supplies across the Atlantic did prove time-consuming and risky and slowed resupply.