Part A: To answer these questions, we will split the War into three parts: Beginning stages, War stage, and the Aftermath.
Beginning stages: In the events surrounding World War I, we could see that war was almost inevitable. A lot of European nations started to build up their military armies, and militarize their nations. Alliances were created based on supportive, population and people groups, as well as strategic standings. The three nations, Austrio-Hungary, the Ottoman Empire, and Germany (as well as some other lesser-known countries) created a pact known as the Central Power. Most other countries (i.e. U.K, Russia, France, Japan, etc) formed the Allies. The War started on the June of 1914 with the assassination of Archduke Ferdinand and his wife Sofia in Serbia, for they had annexed Serbia. War was declared by Austrio-Hungary against Serbia, which soon set off the chain reaction caused by the alliances. Many at this stage also believed that the war would last only a couple of months, but little did they know it would last 4 long bloody years.
Early middle stages: During this stage, we saw a drastic change in the fighting style, as large casualties increasingly piled up, which resulted in what is now commonly-known as trench warfare. In this tactic, opposing armies would dig large networks of trenches protected by sandbags, wires, machineguns, mines, and others, and usually stretched for miles. Land in between opposing armies' trenches were known as "no-man's-land" which was usually a suicide mission resulting from one side charging through this "no-man's-land" to attack the other side. The fighting became especially brutal with the invention and evolving of many mechanisms, including, but not limited too, the creation of machine-guns, tanks, airplanes, and many others. Calvaries were continued to be used in the early parts of the war, but soon gave way to tanks (as they were better protected).
Late middle stages: The late middle stages were crucial in allowing the Central Power to first gain the upperhand but later be crushed by the allies. During this time, the Bolsheviks (Communist Russians) rose to power and were able to defeat the White Russians (non-communist Russians), resulting in the withdrawal of Russia from the war, which allowed Germany to focus her attention on the western-front and their goal, Paris. With the increased amount of troops, supplies, and equipment, the Germans were able to surge through most of the land and appear on the horizon of Paris, making victory seem inevitable. However, the Germans made some costly mistakes outside of Europe that help lead to their downfall.
First, they made the mistake of sinking peaceful merchant & passenger ships, which enraged the US (especially with the sinking of the Lusitania), for they believed that, as a neutral country, they should not be attacked. (Note, Lusitania is owned by the U.K, but there were 128 U.S. citizens on board that died with the result of the sinking of the Lusitania). The second and costly mistake was the Zimmermann Telegram, which was sent from Germany to Mexico to ask for aid in the war, in exchange fore later aid in retaking the lands lost to the US during the Texas Independence and Mexican-American War. This was, as you can say, the "last-straw" for the US citizens, which declared war on the Central Powers.
Last Stages of the War: With the declaration of war by the US, more equipment and troops were poured into Europe, which gave the momentum and the ability to change the tide of war in favor of the US, which led to the defeat of the Central Powers. This led to the creation of what is known as "Wilson's 14-points" treaty, and the creation of the League of Nations. However, while the League of Nations was a good thought, it was not created well enough, and soon, the US and other nations dropped out. The First Great War was over, the Great Depression had arrived, and the Second Great War was on the horizon.
Part B: As stated above, there were many countries that were involved in World War I. It was, after all, called a World War, for it resulted in almost the entire world fighting for one side or the other. Commonly known countries (or the main ones) would be listed underneath:
Central Power:
GermanyAustrio-HungaryBulgariaOttoman Empire
etc.
Allied Powers:
SerbiaRussiaBritain & her
etc.
The magnitude and participation of all of the nations allowed the Great War to earn it's name as the "First World War".
Part C: The Great World War lasted from 28 July, 1914 to 11 November, 1918, which was the span of ~4 years.
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