Answer:
World War I
Step-by-step explanation:
That "incident" caused the first World War... that spread all over Europe. At the end, even Japan and the United States were part of that conflict.
The war itself claimed over 15 million lives, about half military and half civilian.
But the devastation of the war also prepared the perfect conditions for the spread of the 1918 influenza pandemic... which killed anywhere between 50 and 100 million people. That is the real human cost of WWI.
At the end, all parties agreed that there should never be another war as extensive as that one... yet.. only 20 years later... WWII.