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The assassination of Archduke Franz Ferdinand was the catalyst for World War I because it triggered a chain reaction that activated a complex system of European alliances. Austria-Hungary blamed Serbia, issued it a harsh ultimatum, and declared war, which prompted Russia to defend Serbia. In turn, Germany defended Austria-Hungary, and France and Great Britain joined the conflict, leading to a continental war and eventually a global conflict.
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American neutrality in World War I was a significant event primarily because it allowed the U.S. to become the world's leading economic and industrial power by supplying both sides, and its eventual entry into the war tipped the military and financial balance decisively in favor of the Allied Powers, leading to Germany's defeat and shaping the post-war world order.
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The First Battle of the Marne was important because it halted the German advance on Paris, ending their hopes for a quick victory and forcing them to fight a prolonged, two-front war.
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The sinking of the RMS Lusitania was significant because it killed 1,198 people, including 128 Americans, and fueled American public outrage against Germany, contributing to the US decision to enter World War I in 1917.