Humans are the only animals on the planet that kill each other in large numbers. I don't know why. All other animals live quite peacefully with each other and with the planet. Perhaps it's because humans are so clever.
Anyway, millions of people died in wars in the 20th century alone. Quite apart from all the death and suffering, wars also create terrible pollution. Military aircraft use huge amounts of fuel. If an aeroplane (pollution) drops a bomb (more pollution) on an oil refinery, a huge fire starts (yet more pollution).
In the Second World War,
some people dropped nuclear bombs on two
cities in Japan, killing hundreds of thousands of people and
creating a new sort of pollution: radiation. In recent human wars, some
armies used weapons made of a heavy poisonous metal called depleted
uranium. These leave behind another sort of radioactive pollution.
A few years ago, some people called hippies used to say 'make love, not war'. Sadly, many people seem to prefer to make war, not love.