There were three major movements in global politics in the late 19th and early 20th century period that led to the outbreak of WW1. Explain structure of European alliance system on the eve of WW1. Who were the member nations of the Central Powers and of the Allied Powers

The shifting power relations in Europe and the Middle East were closely connected to internal and external conflicts and to furthering the nations’ global reach. As the European nations consolidated their own territories, not all ethnic minorities within them were satisfied with the arrangements, and independence movements often flared up. Diverse nations also joined forces with neighboring nations to strengthen their collective positions. For the smaller countries, having a more powerful partner—in terms of armaments and troops—was essential protection. Some newly consolidated nations, especially Germany, had not only industrialized rapidly but also concentrated on their military buildup. This in turn generated need for natural resources to be obtained from colonies. While all the nations were concerned with activities within and adjacent to their own borders, the global reach of imperial control was also central to the alliances and antagonisms. This was especially the case in Africa, where the “scramble” to establish control saw small footholds transformed into full-fledged colonies.
While the alliance system helped maintain peace, to some extent, the rampant militarization could not go unnoticed for long. The stronger countries began to flex their military muscle. Having a strong ally also encouraged the smaller countries to join the fray once the first declarations had been made. Further, the large territorial extent of some powers meant that fighting in remote outposts played important roles in determining the outcome. Russia extended all the way to the Pacific; the Ottoman Empire, which spanned North Africa, Europe, and Central Asia, had lost territory to Russian in 1878 and still wished to recoup it.
The Central Powers were Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria. This large alliance grew out of an 1879 alliance between Germany and Austria-Hungary, which became the Triple Alliance when Italy joined. War looked inevitable in 1914, after a Serbian nationalist assassinated the Archduke Franz Ferdinand of Austria-Hungary. As the likelihood of war accelerated, the larger countries began to mobilize and sometimes announced their intentions. Germany informed Austria-Hungary, for example, of its intended support should war be declared by Serbia and backed by Russia. The intended Russian allegiance to the Serbian cause included religious motivations, with Tsar Nicholas II invested in protecting Eastern Orthodox believers. After war was declared in 1914, Ottoman participation was kept secret for several months, and Bulgaria joined in 1915.
Operating in opposition to the Central Powers were the Entente powers, primarily the Triple Entente of Britain, France, and Tsarist Russia, soon to be transformed into the Soviet Union. The pronounced German militarism had sustained rumors of war for years prior to the outbreak of hostilities. The Entente’s formation was primarily defensive against the German threat, including its naval capabilities. In 1914, as Russian announced its potential support for Serbia, Germany declared war on Russia. Once France in turn entered, Germany turned west and invaded Belgium. Britain then seemed compelled to join their allies. On the eastern side, the Ottomans declared war on Russia. Although battles were fought across Europe, Asia, and parts of Africa, the United States would enter only in 1917, after the "Great War" had been going on for three years.
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