Keep in mind Europe had almost 100 years of peace before WW1; the war to end all wars.
Have you ever asked yourself why France was so willing to fight Germany?
The Crimean war caused close to 1 million casualties in 3 years.
https://en.m.wikipedia.org/wiki/Crimean_War
The war involved Second French Empire, British Empire, Russian Empire and the Ottoman Empire.
Perhaps there are other examples of major wars, but this one seems significant.
EDIT: Outside of Europe, consider the Taiping Rebellion, with the highest deaths estimates at 30 million (or even higher!). The British and French got involved as well.
Not really: Europe saw plenty of wars after the Napoleonic wars.