We're raised to believe adults shouldn't throw tantrums. Sadly WAY too many people who identify as adults apparently never got that memo. A few minutes on YouTube or Notalways right, or a few days in any service industry will show that.
Wars are generally because somebody powerful wanted something someone else had, and decided to try and take it. Sometimes they manufacure excuses, but ultimately it always boils down to "I want something you have, and you didn't give it to me."
Most wars start as a scheme by a few connected criminals to make money, and usually succeed regardless of which side "wins."