He had contact with the Great White Brotherhood which increased his desire to bring peace to the world. He did many paintings, including the Banner of Peace.

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Nikolai Konstantinovich Rerikh (October 9, 1874 – December 13, 1947) better known as Nicholas Roerich, was a Russian painter, writer, archaeologist, theosophist, philosopher, and public figure.

Roerich, largely by the influence of his wife, Helena, developed an interest in eastern religions, as well as alternative belief systems such as Theosophy. Both Roerichs became avid readers of the Vedantist essays of Ramakrishna and Vivekananda, the poetry of Rabindranath Tagore, and the Bhagavad Gita. He traveled extensively.

The beings from an esoteric Buddhist community in India were said to have told Roerich that Russia was destined for a mission on Earth. That led him to formulate his "Great Plan," which envisaged the unification of millions of Asian peoples through a religious movement using the Future Buddha, or Maitreya, into a "Second Union of the East." There, the King of Shambhala (of the Great White Brotherhood) would, following the Maitreya prophecies, make his appearance to fight a great battle against all evil forces on Earth. 

Roerich reported seeing a metallic oval in the sky over the Tibet; decades later, UFO enthusiasts would claim the Roerich expedition witnessed a "flying saucer".

Source

https://en.m.wikipedia.org/wiki/Nicholas_Roerich

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The Banner of Peace is a symbol associated with the Roerich Pact, the first international treaty aimed at the protection of artistic and scientific institutions and historical monuments.The pact, signed on April 15, 1935, represents a significant milestone in the international effort to safeguard cultural values and heritage. The Banner of Peace was proposed by Nicholas Roerich, a prominent figure in the development of the pact, as a universal emblem to signify protected cultural sites under the agreement.

Source 

https://en.m.wikipedia.org/wiki/Banner_of_Peace

 

Madonna Oriflamma (1932), She is seen holding the Banner of Peace

Nicholas Roerich • Public domain

RootOfAllLight • CC BY-SA 4.0

 

Nicholas Roerich • Public domain

 

Nicholas Roerich • Public domain