I did not completely understand this painting the first time I saw it. It seemed too obscure and scattered and disturbing initially, but I guess that is exactly how Picasso wanted the people to see it. The overall impact of the piece creates a disturbed reality of pain and agony. This impact is formed by figures such as the mother weeping over her dead baby and the dead bodies on the floor and the flames that dominate the painting. Notice how the colors are black, gray and white which I believe symbolize the darkness and evil of war. It is not meant to be pretty and successfully conveys the message that war is ugly, brutal and horrifying. This is one of the most famous artworks that express the atrocities of war and the need for peace, and do so very effectively.

Guernica is a painting by Pablo Picasso, depicting the bombing of Guernica, Spain, by German and Italian warplanes at the behest of the Spanish Nationalist forces, on April 26, 1937 during the Spanish Civil War. The Spanish Republican government commissioned Pablo Picasso to create a large mural for the Spanish display at the Exposition Internationale des Arts et Techniques dans la Vie Moderne (1937) Paris International Exposition in the 1937 World's Fair in Paris.
Guernica shows the tragedies of war and the suffering it inflicts upon individuals, particularly innocent civilians. This work has gained a monumental status, becoming a perpetual reminder of the tragedies of war, an anti-war symbol, and an embodiment of peace. On completion Guernica was displayed around the world in a brief tour, becoming famous and widely acclaimed. This tour helped bring the Spanish civil war to the world's attention.

-Wikipedia