How to stop a war according to Gouthama Buddha? Are there any scriptures discussing on how to prevent wars? 
I mean for example, from the insights of scriptures, if Buddha were alive then how would Buddha have tried to stop World War 2?
There's a description of that here: Why Was the Sakyan Republic Destroyed?
I think the article is based on the canon, but I've forgotten where in the canon I read of it.
Anyway to quote from the article:

When the Buddha learned about this outbreak of hostility, he came there himself. On seeing this revered and eminent person from their royal clan, the warriors of both sides became deeply embarrassed. All of them laid down their weapons and paid homage to him. This was the traditional way of paying respects to the Buddha in those days.
Whenever King Pasenadi of Kosala went to pay respects to the Buddha, he would leave the royal insignia such as his sword, turban, fan, parasol and sandals outside with his attendant before entering the chamber of the Buddha.
The Buddha took the seat prepared for him on the open ground of the riverbank. The Sakyans and the Koliyans paid homage to the Buddha and sat respectfully on one side before him. The Buddha explained to them that their blood was much more valuable than the river-water. They should not spill blood unnecessarily. Instead, they should find a way to share the water peacefully. The Buddha was a peacemaker filled with compassion. He did not consider it proper that human life should be lost for the sake of trivial material gains.

And it goes on -- you can read the article for more.
The eventual outcome was that the Sakyan clan was massacred.