My answer won't really answer your question because the first answer says it all. What I want to tell you is that if you ever get to Spain, go see the painting. What strikes you first is the enormous size of the painting which dominates one whole wall in the museum; the scope of its size is unbelievable. Then you start looking at the details of the painting itself, and Picasso's genius is so clear. In this whole huge painting, no matter where you look, you see pain and suffering. War was the horror inflicted on his homeland, and you can feel his deeply felt grief and regret that such a terrible thing as a CIVIL WAR between the peoples of his own country should ever happen on Spanish soil. The painting is overwhelming in its grief and depiction of war.
The motivation for Picasso's work is the horrific state of war. From Picasso's point of view, the Spanish Civil War was one that begged to be depicted. The subject of the painting is the consequence of this battle, a canvass of death and suffering that tears the mask off of war and reveals it for the savage cruelty it is. The battle between freedom fighters and the Fascist government was one in which Picasso saw freedom and human expression in its most elemental form:
The Spanish struggle is the fight of reaction against the people, against freedom. My whole life as an artist has been nothing more than a continuous struggle against reaction and the death of art. How could anybody think for a moment that I could be in agreement with reaction and death? ... In the panel on which I am working, which I shall call Guernica, and in all my recent works of art, I clearly express my abhorrence of the military caste which has sunk Spain in an ocean of pain and death.
In this paradigm, the motivation for the painting is to depict the realities of this struggle. Picasso does not romanticize this struggle, but rather brings out the idea that the level of horror and atrocity committed in the name of war is a reality that must be acknowledged entirely on its own. The painting is one that depicts the horror and the "abhorrence" of how the military can savagely rupture the bonds that exist between individuals and the military. In this, the picture emerges.