On June 24, 1941, two days after Nazi Germany invaded the Soviet Union, General Francisco Franco announced the creation of a Spanish volunteer unit “to fight Bolshevism” that would eventually grow to include some 48,000 troops.

The División Azul, or Blue Division, was incorporated into the German Armed Forces as the 250th Division of the 16th Army and fought on the Russian Front. Its name came from the blue shirts worn by the Falange, the political movement that Franco took over, but its soldiers wore German Wehrmacht uniforms. In 1944, with the United States in the war and Soviet troops advancing on Germany, it was disbanded, although some Spanish soldiers fought in the defence of Berlin in 1945.

It remains one of the last taboos of the Franco era: there is no mention of it in school textbooks; of the countless films made about the Civil War era no director has dared touch it; and there have been no television documentaries either.

That said, recent years have seen a growing number of books about the unit, the latest of which is Jorge Martínez Reverte’s División Azul: Russia, 1941-44.

Reverte’s own father, Jesús Martínez Tessier, a well-known journalist, fought in the División Azul, but never talked about his time in Russia. “Not because he was ashamed, or felt that he had done anything wrong,” says Reverte in the book’s introduction, “but because the experience was traumatic.”

When Reverte’s father died a few years ago, he left behind his diaries and memoirs, which deal with his time in the División Azul. Like many who had fought on the Republican side in the Spanish Civil War, Reverte says that his father enlisted to fight in Russia as a way of proving his loyalty to the new regime.

But the majority of volunteers, says Reverte, joined “because they were possessed by a destructive and combative urge that they had developed during the wars in Africa, something that today is hard to understand.”

Reverte has written extensively about the Spanish Civil War, and describes the División Azul as “one of the most shameful chapters in Spanish military history.”

“Franco took sides in the Second World War with Hitler, who was responsible for one of the worst genocides in history; that said, he was fighting somebody responsible for another genocide: Stalin,” says Reverte.

Reverte nevertheless says that as he researched further into the subject, his understanding of the historical complexities of the División Azul began to deepen, particularly regarding the role of the Catholic Church.

“I was unaware that the driving force behind the creation of the División Azul was Catholic-Nationalist. The Italian, and even the German Catholic Church protested at what the Nazis were doing in Russia, but the Spanish Church never said a word, despite knowing about the holocaust.”

Aside from his father’s testimony, Reverte has interviewed survivors and the families of those who fought in the División Azul. He also visited the battlegrounds and followed the 900 kilometers from Poland into Russia that the Spanish troops had to march.

“I have tried to provide an objective and balanced version of events,” he says, “I travelled to the areas where the División Azul fought so that I could understand the harsh conditions they had to fight under, and to meet the people who live there: without that trip I couldn’t have written the book.”

Almost 5,000 members of the División Azul died fighting for Nazi Germany, but after the war, with Spain soon to become a NATO ally, Franco’s support of Hitler was overlooked, and the División Azul was rarely mentioned.

Franco insisted that the Division Azul be used only against the forces of Bolshevism, which meant it was sent to the Russian Front, thus avoiding any conflict with the Allied Forces. This decision doubtless made it easier for the Allies to overlook Franco’s support for Hitler after the war.

Hitler referred to the division as “equal to the best German units”. Later in the war, when Hitler considered an invasion of Spain to remove Franco and replace him with Agustín Muñoz Grandes, he decided against it, saying “The Spaniards are the only tough Latins. I would have a guerrilla war in my rear.”

The División Azul cast a long shadow; one that extended beyond the death of Franco. Many high-ranking officers in the Spanish Army in 1960s and 1970s had served in the División Azul, and they were unhappy at the prospect of a return to democracy. Some of the key figures behind the failed coup on February 23, 1981 had served in the unit. Amongst them were generals Alfonso Armada and Jaime Milans del Bosch. At the same time, other División Azul veterans, for example José Luis Aramburu Topete, head of the Civil Guard, and José Gabeiras remained loyal to the democratic government.

For several years after the death of Franco, members of the División Azul continued to march in Spain’s annual October 12 Armed Forces Day parade. In 2004, the new Socialist Party government invited three veterans of the División Azul to march alongside members of the Army of the Second Republic that lost to General Franco in the Civil War.