Wat is the main idea of "War and Peace"?

Yikes, that's a big book to summarize in a few sentences. In any case I hope this helps,

The work is epic in scale and is regarded as one of the most important works of world literature.[1][2][3] It is considered Tolstoy's finest literary achievement, along with his other major prose work Anna Karenina (1873–1877).

War and Peace delineates in graphic detail events surrounding the French invasion of Russia, and the impact of the Napoleonic era on Tsarist society, as seen through the eyes of five Russian aristocratic families.