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Iran-Iraq War [2]
Casualties and losses
320,000–720,000 soldiers and militia killed (Iran)
150,000–375,000 soldiers and militia killed (Iraq)
Economic loss of more than $500 billion (per side)
100,000+ civilians killed on both sides
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World War II was the deadliest military conflict in history. Over 60 million people were killed, which was over 2.5% of the world population. [3]
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[1] This is also clearly visible in the citation of the GP comment, but i provide more examples.