If each and every country in this world keep a "full stop" to the procurement and development of military weapons, there will be no war.
But " if " a single country keeps a secret comma and starts developing its weapons it will become dangerous. For this " if " factor, millions and trillions of money has to be spent by every country for "no" good reason at all.
As regards this write, the aliens from other planets attacking the Earth must not be considered. If the aliens and their possible invasion are considered then the trillions of money spent by countries to develop their military have their own good reasons.