Final answer:
World War I was caused by a complex mix of Militarism, Alliances, Imperialism, and Nationalism, with the assassination of Archduke Franz Ferdinand as the immediate trigger. The entry of the US into the war was precipitated by the resumption of unrestricted submarine warfare by Germany and the Zimmermann Telegram. The sinking of the Lusitania exemplified Germany's violation of international law and contributed to bringing the US into the conflict.
Step-by-step explanation:
The long-term causes of World War I can be summarized with the acronym MAIN: Militarism, Alliances, Imperialism, and Nationalism. Militarism involved the arms race among European nations, with each seeking to outdo the others in military strength. Alliances between countries, particularly the Triple Entente (France, Russia, and the United Kingdom) and the Triple Alliance (Germany, Austria-Hungary, and Italy), created a situation where conflict between two countries could escalate into a wider war. Imperialism led to rivalries over colonies and resources, while Nationalism caused tensions both within and between nations.
The assassination of Archduke Franz Ferdinand acted as the immediate catalyst for World War I, as it led to a series of diplomatic failures and ultimatums that ultimately involved the major powers through their complex system of alliances. Austria-Hungary's declaration of war on Serbia set off a chain reaction, with Russia mobilizing to support Serbia, Germany declaring war on Russia, and consequently on France, and the United Kingdom entering to support Belgium, which was violated by German troops.
Two key events prompted the U.S. to enter WWI: the resumption of unrestricted submarine warfare by Germany and the Zimmermann Telegram, in which Germany proposed a military alliance with Mexico against the U.S. International law required a U-boat to surface and give warning before attacking a merchant ship to allow for the safety of civilians, but Germany abandoned this practice in favor of unrestricted warfare to cut off supplies to their enemies. This led to the sinking of the Lusitania, a British passenger liner, by a German U-boat, killing 1,198 people, including 128 Americans, which significantly shifted American public opinion against Germany.