In the century before World War One, Europe experienced an era of relative peace and prosperity. From 1815 - 1914, European nations used alliances to maintain the balance of power, so that no single country could become too powerful. A number of wars were fought during this period, but these were limited in scale. New powers arose (Germany, Italy) while older ones faded (Ottoman Empire, Austria-Hungary), but overall, there was a sense of stability.
However, powerful forces were developing that would push Europe’s nations into a major conflict that would ultimately draw in much of the world. Industrialization increased the capacity of nations to produce the tools of war on a mass scale. Thanks to advances in medicine, food cultivation and transportation, the population of Europe tripled, expanding the manpower pool available for military service. Nationalism was on the rise, driven in the larger nations by competition for overseas colonies. In addition, minority populations within the larger powers (especially Austria Hungary and the Ottoman Empire) increasingly demanded the right to govern themselves. The system of alliances between the larger nations expanded to include additional agreements between major powers and lesser nations.
Thus, by 1914 every major European power possessed a growing industrial economy and the ability to field a massive army of willing patriots, and was connected to a web of obligations and guarantees that entangled nearly every country in Europe.
Over the course of the 19th century, much of the world came under European rule. Competition for colonies greatly accelerated, driven by the need for both raw materials and markets to feed Europe’s growing industrial economies. With the exception of the United States, Central and South America, the Ottoman Empire and Japan, most of the Earth’s population was under the control of a European power. When war came, territories in every corner of the globe would send manpower and resources to Europe, or would themselves become battlegrounds.