War .... any kind of war for any reason.
The idea that the only way to for us to settle disputes or disagreements with others is to organize ourselves into competing camps, make fancy plans and use all our technical knowledge and technology to kill one another.
Our ancient prehistorical ancestors had the same logic thousands of years ago and used sticks, spears, rocks and their bare hands.
Today, we use drones, machine guns, army tanks, naval ships and nuclear weapons.
The technology has changed ... but our mentality is still the same.