Picasso painted it after the bombing of civilians by German air forces who were allied with Franco. I believe it was Germany's opportunity to practice their new form of ariel warfare. Guernica was bombed on market day, when the town was full of families who travelled to town. It happened during several waves of attacks, not just one. Men, women and children who were running into the countryside to hide were hunted down by pilots and machine gunned.

If I remember correctly, Picasso had the painting displayed at a large show, and was standing next to it when a German officer said to him "did you do this"? to which Picasso replied "No, YOU did".

When you look at the painting from all angles, you see the symolism of what happened to those innocent people at the market. A horse blown up. A screaming mother holding her child killed by shrapnel. People fleeing. Death skulls. It shows the horror.

As for changing history, I don't know. I remember when Colin Powell made his speech at the United Nations about why we must go to war with Iraq. A copy of Guernica happened to be in back of where he was making his speech, so they covered it up with a cloth. Make of that what you will.