The assassination of the Austro-Hungarian heir shook up the unstable peace between the nations of Europe. The series of retaliations that followed would draw the strongest military powers in Europe into large-scale conflict as they sought to support their alliances, grow their empires, and crush their rivals.

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Prior to the assassination, Austria-Hungary already had a rocky relationship with the nations of the Balkan Peninsula. The Archduke’s assassin was a Serbian national who had worked with terrorist groups to further the cause of a united empire for the region’s Slavic people. This gave Austria-Hungary the opportunity they had been waiting for to declare war on Serbia.

Russia’s mobilization of troops in the summer of 1914 sent a clear signal that they were preparing to strike against Austria-Hungary in order to defend their Serbian allies. Germany, a member of the Triple Alliance with Austria-Hungary, was obliged to declare war against Russia to help defend their ally.

Belgium had been a historically neutral nation and, therefore, depended upon alliances with France and Great Britain for defense. The “Great War” began in large part because of such alliances and defensive pacts. Starting with the Austria-Hungarian invasion of Serbia, the European superpowers were drawn in one after another.

Despite eventually falling to the Germans, the Belgians held out for almost three weeks against the invasion. This time played a crucial role in allowing the French and the British to get troops in place to block the German advance. The Battle of the Marne was a decisive victory for the Allies (Britain and France) which kept the Germans from reaching Paris.

While World War I escalated rapidly across Europe, the ability for armies to make significant gains fizzled out almost as quickly. Trench warfare was a slow and plodding tactic that resulted in a virtual stalemate between the two warring alliances.

Hundreds of thousands of men lost their lives in these two battles. Despite the tremendous loss of life, there was little net gain for either side. The use of new weapons like poison gas and heavier artillery added to the massive casualty numbers.

The German U-boat submarines not only allowed Germany to break through British blockades of German ports, but they also created a virtually invisible blockade of their own, preventing supplies and munitions from entering Great Britain by sea.

President Woodrow Wilson held tight to the notion that America should not engage militarily in “The Great War”. He faced heavy opposition from an American public, many of whom were immigrants with strong, personal ties to their European heritage. Despite staying out of the fighting, America would profit from a boom in trade and new loan opportunities with the Allied Powers.

Outrage over the loss of American life in the Lusitania attack in the spring of 1915 fueled Congress’s decision to bolster the American military the following year. The story of the Lusitania painted the picture of a ruthless German navy willing to attack civilian craft; however, it has since been proven that the Lusitania was also carrying a stockpile of munitions bound for Britain. While there were other factors that led to America breaking its neutrality, the sinking of the Lusitania undeniably played a role in stirring the American public’s desire for action.

Arthur Zimmerman, the German ambassador to Mexico, promised Mexico money as well as their “lost territory in New Mexico, Texas, and Arizona” in exchange for Mexico’s willingness to launch an attack the United States. A British intelligence officer intercepted Zimmerman’s telegram and publicized its contents. The Zimmerman Note, also known as the Zimmerman Telegram, played a key role in building the American public’s support for entering the war.

The Selective Service Act was a crucial piece in America’s ability to field a strong military. About 1/8 of the American soldiers that participated in World War I were drafted into service.

The American naval convoy was an effective countermeasure to the German U-Boats prowling the Atlantic. By some estimates, the convoy system resulted in a decrease in lost cargo by about 2/3. The convoys also prevented the U-boats from taking the lives of any Europe-bound U.S. troops.

Following the Bolshevik revolution, Russia’s new leader, Vladimir Lenin, was intent on setting up the nation as a Communist state. The war was not seen as an effective or meaningful way to advance his plans, so Lenin signed the Treaty of Brest-Litovsk, gave up Poland and Ukraine to Germany, and left the war.

The Germans were able to use the troops that had been allocated to the fight against Russia to surge against the Allies in western Europe. This push broke the ongoing stalemate on the Western Front and allowed Germany to finally get within striking distance of Paris.

Under the command of General John Pershing, the American Expeditionary Force successfully turned the tide of war in favor of the Allied Powers with hard-fought victories over the German offensives on the Marne River, in Belleau Wood, and in the Argonne Forest. Without the American support, it would have been less likely that the British and French forces could have repelled the German invasion.

Facing a hopeless military situation, Germany requested an armistice on October 4, 1918. American president Woodrow Wilson was open to the idea, but he made it clear that made it clear that the Allies would only negotiate with a democratic Germany, not an imperial state with a military dictatorship. Armistice negotiations opened on November 7th, and the war came to an end 4 days later.

Part of President Woodrow Wilson’s “Fourteen Points” plan for peace, the League of Nations was a primitive version of the modern United Nations. Ultimately, the founding principles were too weak and the organization was unable to overcome the member nations’ individual interests; this was especially ironic given the fact that Wilson’s “Fourteen Points” was focused on the idea of national self-determination.

Germany was harshly punished for their role in World War I, which was how the European nations of the Allied Powers wanted it. They wanted assurances that Germany would not simply rearm and repeat the process. Conversely, President Woodrow Wilson had a difficult time gaining support for the Treaty in America partly because the included League of Nations (his own proposal) tied America to the affairs of other nations.