In today's context of standing armies, the idea of a country taking months to mobilize for war seems rather foreign. How did countries go about mobilizing? What sort of plans did they have in place to deal with the workers who would no longer be at their jobs? What were the differences in the process between the more industrialized and agrarian nations?
In each nation, it was the duty of the General Staff to draw up mobilization plans in the event of war. These would include all of the mechanism for getting the troops into fighting order and moving to the front as fast as possible. Germany had a famously intricate mobilization plan leading up to world war one, which famously detailed down to the minute the plans for mobilizing the nation and preparing for the strategically vital invasion of France. Russia and France, too, had very elaborate plans.
All the powers relied on rail mobilization in the event of war. During mobilization proper reservists would be called and would then report to their respective regimental depots before embarking by train to their destination. The railroads had pre-established relationships with the European powers. The workers on these rails were familiar with what their wartime duties would entail. The governments had subsidized the rail companies and included double tracks and strategic lines to ensure a quick and orderly mobilization.
Britain had a somewhat stranger mobilization process, as they were an island lacking any boarder with the other powers. Britain would man her reserve vessels, run the BEF’s troops to the shore and disembark to land her troops in France, where they could be moved by French rails to the front.
Source: History of World War I: War and Response, 1914-1916