No matter what - peace will never perish, and as it will never perish, it will never circulate fully.
There are three matters as to why peace doesn't last.
Hate → Hate is always found, in every corner. If hate is found, jealousy is found. And where jealously is found, there will be arguments.
The world is not always fair → As much as we know, poverty and "dark societies" are out there suffering from starvation and much more. Can these people live in peace without support? No, they cannot. They need support and help, but greed had overcome the souls of mankind.
War → Whether it last 3 days, or 3 years, war is always found. Peace is found after a war, but sometimes, wars don't end with fair agreements, causing more war.
What can I say? Life is life, peace isn't found everywhere.
Social stratification is basically the separation of society into different classes. Karl Marx thought this system will inevitably fall and a new Communist society formed by only the middle class will take its place.
