What do you think was the chief motive behind the Guernica painting?

The painting was commissioned by the Republican government of Spain, to denounce the bombing of Guernica.

Pablo Picasso was commissioned to paint Guernica by the Republican government for an International exposition in Paris in 1936. This government was still internationally recognized as the government of Spain as the Civil war had just started. The painting and this explanation can be viewed at the Museo Reina Sofia in Madrid.