John Hopkins

He jumps as an almighty crash sounds from up ahead.

A building at the end of the street has collapsed, sending a shower of sparks in all directions.

He retraces his steps and tries another road, but is waved back by a team of firemen.

Even they cannot get into the town center, they tell him.

It is an inferno.

Defeated, George retreats to where the cars are parked, takes out his notebook, and begins to record what he has seen.

The world must know of the horror that has been inflicted on Guernica.

To this day, the number of people killed during the bombing of Guernica remains unknown.

Republican estimates from the time suggest that more than 1,500 civilians perish, though modern historians consider the true figure to be lower.

But it is far from a purely Francoist attack.

The planes used are exclusively German and Italian, with the Luftwaffe masterminding the whole assault.

Dropping high explosives to destroy roofs and buildings before unleashing incendiary devices to create a firestorm, they then strafe fleeing civilians with gunfire, the whole operation acting as a dress rehearsal for similar atrocities in World War II.

George Stier's eyewitness account of the aftermath is carried by news outlets across the world.

This almost unprecedented aerial attack on a civilian population provokes outrage and inspires Pablo Picasso's famous mural depicting the carnage.

The bombing crushes the ability of defenders within the town to resist the Francoist advance, and it falls at the end of April.

By the end of June, Franco is in control of the entire Basque region.

As the center of Spain's heavy industry, its loss represents an immense blow to the Republican war effort.

Though they chalk up some victories in the summer of 1937, in October Asturias, the nation's coal mining region, is conquered by Franco as well.

By now the Francoists have managed to block or impede access to all of Spain's Mediterranean ports with the help of their fascist allies.

The provision of supplies and arms to the Republicans, the latter mostly acquired through intermediaries to get around the non-intervention treaty, is now a severe logistical challenge.