The causes of the Economic Boom of the 1920s were the Republican government's policies of Isolationism and Protectionism, the Mellon Plan, the Assembly line and the mass production of consumer goods such as the Ford Model T Automobile and luxury labor saving devices and access to easy credit on installment plans.
I'm pretty sure its the first one
The Austrian Archduke Franz Ferdinand was visiting Serbia, which was an extremely controversial decision considering the relationship between the Austrians and the Serbians was very rough, as Serbia was then owned by the Austro-Hungarian empire, which caused civil unrest in Serbia. While Ferdinand was driving down the street to visit families who were victims of a horrible bombing that was actually an attempted assassination on Ferdinand himself, Gavrilo Princip, shot him and his wife. As a result, the Austro-Hungarian Empire devised a list of policies that really served to ruin the Serbian economy, as well as put Serbia in as unstable a condition as possible. Russia, who at the time was a very strong ally of Serbia, then mobilized its army because of Austria's then reaction to the assassination of Ferdinand. Germany then allies with the Austrio-Hungarian Empire, signified by the "blank check" Kaiser Wilhelm II gives to the Austrians, authorizing any decision the Austrians decide to make regarding the Balkan Crisis. Because France was allies with Russia at the time, and the Franco Prussian War destroyed the relations between France and Germany, Germany declared war on France, who immediately responded by forging an alliance with both Great Britain and Russia.
As Europe grew more and more connected, western Europeans believed that the way to create a better future for them and their families was to leave Europe and come to America where they'd have better opportunities and the idea of a new world that was being founded by whoever came. That led to a lot of western Europeans, British, Spanish, Irish, migrating from Europe to America in the late 1800's.
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