What was the root of the idea in having a united nations?

there had always been a spirit of peace in europe.
earlier organizations had attempted to stop wars.
the americans had introduced plans for an international group of nations after world war i.
during world war ii, the idea was being planned for this assembly.

The third option.

WWI with President Wilson tried to Introduce a League of Nations fot international peace which is what this question is addressing.

Leaders in the Roman republic are elected into their positions and that influenced how the U.S. Constitution works. The Roman republic had a system of checks and balances to prevent one person from having too much power. The way the Roman republic works is very similar to the way the U.S. Constitution works today.