Mobilization is the act of preparing military forces and logistics for war, through heightened readiness status and where applicable by the calling up of reserves. True mobilization (whether complete or partial) generally occurs only when war is imminent or has broken out. According to wikipedia, the word mobilization was itself first used in a military context to describe the preparation of the Prussian army during the 1850s and 1860s.
Canada has only mobilized twice, for the First World War in 1914 and again just prior to the Second World War in 1939.
In the 1800s, the introduction and/or existence of railway transport, telegraph communication, conscription and large standing armies in Europe essentially changed the character of modern warfare and the ability of nations to prepare for war. In the fragile diplomatic climate that characterized Europe as the 20th Century approached, intricate war planning was common, and detailed plans for mobilization had to be drawn up well in advance to avoid chaos. So complicated was the process of turning out the military that general mobilization of a nation's military was seen as an act of war in itself.
First World War
The United Kingdom was the only major power in England without conscription; Canada also relied on volunteers. Reservists in Canada were not liable for full time military service; in Europe however, calling out reserves - i.e. placing them on full time active service - was a major method of force generation. Peacetime economies did not generally allow for large standing armies, so shortfalls of men needed in time of war were made up by mobilizing the part time reservists. However, the ability to move millions of partially trained men was hampered by the need for intricate war plans, involving railroad schedules and a preconceived notion of where the men were to be employed.
Germany's war plans were all predicated on a war with Russia and France simultaneously, with the "Schlieffen Plan" outlining a quick strike against France, delivering a knockout blow through neutral Belgium, and then turning attention to Russia. Russia, for her part, assumed that any war on their frontiers would involve both Germany and Austria-Hungary. On 28 Jul 1914, Czar Nicholas II of Russia ordered a partial mobilization in response to Austria-Hungary's position on Serbia. His military leaders, however, advised that partial mobilization would prove chaotic. Despite the desire to avoid war with Germany, Russia mobilized its forces on the frontier with Germany also. Germany promptly declared war on Russia, and itself mobilized. Again, hampered by inadequate plans for partial mobilization or war against Russia only, Germany mobilized against France as well and put into effect the Schlieffen Plan.
Canada was compelled to go to war when Britain did. What was notable about Canadian mobilization was that a field force was created from scratch rather than using the existing regimental structures as a framework. The Militia Minister, Sir Sam Hughes, created the Canadian Expeditionary Force from whole cloth 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 for the duration of the First World War, generating over 250 numbered infantry battalions and scores of units from the cavalry and supporting arms. In 1920 the Otter Committee 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 the "call to arms" of ancient Scottish clans assembling for battle than a modern, industrialized nation preparing for war.
Second World War
On 1 September 1939 the German invasion of Poland prompted both France and Britain to declare war; mobilization was slow and the Allies were unable to carry out anything but minor operations in the west before Poland had been overrun.
Canada, having gained a measure of political autonomy between the wars, actually carried out a partial mobilization on 25 August 1939 in anticipation of the growing diplomatic crisis. On 1 September 1939, the Canadian Active Service Force (a corps-sized field force of two divisions) was mobilized despite war not being declared by Canada until 10 September 1939. Only one division went overseas in December 1939, and the government hoped to follow a "limited liability" war policy. When France was invaded in May 1940, the Canadian government realized that would not be possible and mobilized three additional divisions, beginning their overseas employment in August 1940 with the dispatch of the 2nd Canadian Infantry Division (some units of which were deployed to Iceland and Newfoundland for garrison duty before moving to the UK). Canada also enacted the National Resources Mobilization Act in 1940, which among other things compelled men to serve in the military, though conscripts mobilized under the NRMA did not serve overseas until 1944. Conscripts did, however, serve in the Aleutian Islands in 1943 though the anticipated Japanese defence never materialized due to the evacuation of the enemy garrison before the landings. Service in the Aleutians was not considered "overseas" as technically the islands were part of North America.
Mobilization during the Cold War
Full scale mobilizations were not enacted by the two superpowers, the USA and the USSR, or their allies, during the Cold War, as the era was characterized by "war by proxy". While many leading nations participated in military actions, some of significant size, national mobilization in the historical sense never occurred. Examples of this would be US involvement in Vietnam (the majority of US soldiers in that conflict were volunteers), though their enemies may be said to have mobilized, as well as the Falklands/Malvinas war between Britain and Argentina.
Canada was compelled to keep a standing army in Europe, consisting of a mechanized brigade. Gwynne Dyer noted that any conflict in Europe between the superpowers would have been a "come as you are war." The astronomical price of modern military equipment precluded the ability to stockpile large amounts in peacetime, or the ability to produce replacements quickly during a protracted war.