Mobilization
History: 

Roman Republic: The Roman Republic was able to mobilize at various times between 6% (81–83 BCE) to as much as 10% (210s BCE) of the total Roman population, in emergencies and for short periods of time. This included poorly-trained militia.

Modern era: The Confederate States of America is estimated to have mobilized about 11% of its free population in the American Civil War (1861–1865). The Kingdom of Prussia mobilized about 6–7% of its total population in the years 1760 and 1813. The Swedish Empire mobilized 7.7% in 1709.
Armies in the seventeenth century possessed an average of 20,000 men. A military force of this size requires around 20 tons of food per day, shelter, as well as all the necessary munitions, transportation (typically horses or mules), tools, and representative garments. Without efficient transportation, mobilizing these average-sized forces was extremely costly, time-consuming, and potentially life-threatening. Soldiers could traverse the terrain to get to war fronts, but they had to carry their supplies. Many armies decided to forage for food; however, foraging restricted movement because it is based on the presumption that the army moves over land possessing significant agricultural production.
However, due to new policies (like conscription), greater populations, and greater national wealth, the nineteenth-century army was composed of an average of 100,000 men. For example, in 1812 Napoleon led an army of 600,000 to Moscow while feeding off plentiful agricultural products introduced by the turn of the century, such as potatoes. Despite the advantages of mass armies, mobilizing forces of this magnitude took much more time than it had in the past.
The Second Italian War of Independence illustrated all of the problems in modern army mobilization. Prussia began to realize the future of mobilizing mass armies when Napoleon III transported 130,000 soldiers to Italy by military railways in 1859. French caravans that carried the supplies for the French and Piedmontese armies were incredibly slow, and the arms inside these caravans were sloppily organized. These armies were in luck, however, in that their Austrian adversaries experienced similar problems with sluggish supply caravans (one of which apparently covered less than three miles per day).
Not only did Prussia take note of the problems in transporting supplies to armies, but it also took note of the lack of communication between troops, officers, and generals. Austria's army was primarily composed of Slavs, but it contained many other ethnicities as well. Austrian military instruction during peacetime utilized nine different languages, accustoming Austrian soldiers to taking orders only in their native language. Conversely, in an effort to augment the efficacy of the new "precision rifle" developed by the monarchy, officers were forced to only speak German when giving orders to their men. Even one Austrian officer commented at Solferino that his troops could not even comprehend the command, "Halt." This demonstrates the communicative problems that arose quickly with the advent of the mass army.

Mobilization in World War I: Intricate plans for mobilization contributed greatly to the beginning of World War I, since in 1914, under the laws and customs of warfare then observed (not to mention the desire to avoid compromising national security), general mobilization of one nation's military forces was invariably considered an act of war by that country's likely enemies.
In 1914, the United Kingdom was the only European Great Power without conscription. The other Great Powers (Austria-Hungary, Italy, France, Germany and Russia) all relied on compulsory military service to supply each of their armies with the millions of men they believed they would need to win a major war. France enacted the "Three Year Law" (1913) to extend the service of conscripted soldiers to match the size of the German army, as the French population of 40 million was smaller than the German population of 65 million people. The Anglo-German naval arms race began, sparked by the German enactment of the Second Naval Law. Each of the Great Powers could only afford to keep a fraction of these men in uniform in peacetime, the rest were reservists with limited opportunities to train. Maneuvering formations of millions of men with limited military training required intricate plans with no room for error, confusion, or discretion after mobilization began. These plans were prepared under the assumption of worst-case scenarios.
For example, German military leaders did not plan to mobilize for war with Russia whilst assuming that France would not come to her ally's aid, or vice versa. The Schlieffen Plan therefore dictated not only mobilization against both powers, but also the order of attack—France would be attacked first regardless of the diplomatic circumstances. To bypass the fortified Franco-German frontier, the German forces were to be ordered to march through Belgium. Whether or not Russia had committed the first provocation, the German plan agreed to by Emperor William II called for the attack on Russia to take place only after France was defeated.

Similarly, the Russian Stavka's war planning assumed that war against either Austria-Hungary or Germany would mean war against the other power. Although the plan allowed flexibility as to whether the main effort would be made against Germany or Austria-Hungary, in either case units would be mobilized on the frontiers of both Powers. On July 28, 1914, Tsar Nicholas II of Russia (William's cousin) ordered partial mobilization against Austria-Hungary only. While war with Austria-Hungary seemed inevitable, Nicholas engaged in a personal dialogue with the German Emperor in an attempt to avoid war with Germany. However, Nicholas was advised that attempts to improvise a partial mobilization would lead to chaos and probable defeat if, as pessimists on the Russian side expected, no amount of diplomacy could convince the Germans to refrain from attacking Russia whilst she was engaged with Germany's ally. On July 29, 1914, the Tsar ordered full mobilization, then changed his mind after receiving a telegram from Kaiser Wilhelm. Partial mobilization was ordered instead.
The next day, the Tsar's foreign minister, Sergey Sazonov once more persuaded Nicholas of the need for general mobilization, and the order was issued that day, July 30. In response, Germany declared war on Russia.
Germany mobilized under von Moltke the Younger's revised version of the Schlieffen Plan, which assumed a two-front war with Russia and France. Like Russia, Germany decided to follow its two-front plans despite the one-front war. Germany declared war on France on August 3, 1914, one day after issuing an ultimatum to Belgium demanding the right of German troops to pass through as part of the planned pincer action of the military. Finally, Britain declared war on Germany for violating Belgian neutrality.
Thus the entangling alliances of the Triple Alliance and the Triple Entente directed the intricate plans for mobilization. This brought all of the Great Powers of Europe into the Great War without actually utilizing the provisions of either alliance.

The mobilization was like a holiday for many of the inexperienced soldiers; for example, some Germans wore flowers in the muzzles of their rifles as they marched. Trains brought soldiers to the front lines of battle. The Germans timetabled the movements of 11,000 trains as they brought troops across the Rhine River. The French mobilized around 7,000 trains for movement. Horses were also mobilized for war. The British had 165,000 horses prepared for cavalry, the Austrians 600,000, the Germans 715,000, and the Russians over a million.
Britain's Dominions including Australia, Canada, New Zealand and South Africa were compelled to go to war when Britain did. However, it was largely left up to the individual Dominions to recruit and equip forces for the war effort. Canadian, Australian and New Zealand mobilizations all involved the creation of new field forces for overseas service rather than using the existing regimental structures as a framework. In the case of Canada, the Militia Minister, Sir Sam Hughes, created the Canadian Expeditionary Force by sending telegrams to 226 separate reserve unit commanders asking for volunteers to muster at Valcartier in Quebec. The field force served separately from the Militia (Canada's peacetime army); in 1920 the Otter Commission was compelled to sort out which units would perpetuate the units that served in the trenches—the CEF or the prewar Militia. A unique solution of perpetuations was instituted, and mobilization during the Second World War did not repeat Sir Sam Hughes' model, which has been described by historians as being more closely akin to ancient Scottish clans assembling for battle than a modern, industrialized nation preparing for war.

"Colonials" served under British command though, perhaps owing to the limited autonomy granted to the Dominions regarding their respective mobilizations, the Dominions eventually compelled the British government to overrule the objections of some British commanders and let the Dominion forces serve together instead of being distributed amongst various British divisions. The "colonials" would go on to be acknowledged by both the British and German high commands as being elite British units. In May 1918, when command of the Australian Corps passed from William Birdwood to John Monash, it became the first British Empire formation commanded totally free of British officers.
On May 23, 1915, Italy entered World War I on the Allied side.