Guernica is undoubtedly one of Picasso's most famous and most touching paintings. Pablo Picasso finished painting Guernica in 1939.
As you say in your question, Guernica is very much connected to the Spanish Civil War. The Spanish Civil War took place between 1936 and 1939 and saw the left-wing Popular Front (Frente Popular) fight against the right-wing Nationalist Faction (Bando Nacional).
Guernica was a town in the Basque area of Spain that was infamously bombed by the German Luftwaffe, as the Germans were allies of Francisco Franco, the leader of the Spanish Nationalist faction. The town of Guernica was rumored to play a significant part within the resistance movement, which is why it was chosen as a target for this attack. What made this attack so horrible was the fact that so many innocent civilians died during this attack, as the bombs had been aimed straight at the town center on a very busy market day. By attacking innocent civilians, the Nationalist Faction had hoped to make a point and to deter other rebels from pursuing their rebellion in the future.
It is this unnecessary death of innocent civilians which forms the center of Picasso's oil painting Guernica. In the paining, you can see people's faces distorted by fear and pain. For example, you can see a grieving woman holding her dead child. This clearly shows the pain that the Spanish Civil War inflicted unnecessarily on innocent people. The painting also shows the horror of war as it would have been experienced by the soldiers, as it also depicts a dead, dismembered soldier under a horse.