At 4:45 a.m. on September 1, 1939, Adolf Hitler’s Nazi forces opened fire on Poland, marking the first salvos of what would soon escalate into the biggest and deadliest war in history. Across Great Britain in those early morning hours, another significant mobilization was underway.

At 5 a.m., children assembled at Myrdle School in Stepney, name tags around their necks. The students at St. Dominic’s Infant School in Hackney huddled with their belongings at 7 a.m. Over 800,000 school-age children living in targeted urban areas across the country gathered at their schools for evacuation. On September 3, Great Britain and France declared war on Germany, beginning World War II.

In all, Operation Pied Piper would see some 1.5 million people, including 800,000 children, evacuated from urban centers over the course of just three days. They were relocated to other mostly rural communities considered safer from the threat of German bombs.

“We have during the past few days seen the most remarkable movement of the civil population ever recorded in history,” announced Member of Parliament James Ede in an address to the House of Commons on September 14, 1939.

Despite the swift mobilization, the evacuation “was not a knee-jerk reaction to the situation in Europe,” says Dr. Penny Starns, author of Blitz Families and an Honorary Research Fellow at the Centre for Health, Humanities and Science at the University of Bristol. In fact, it wasn’t even the country’s first evacuation scheme, she says, explaining that plans for evacuation were made over a century earlier in Dorset under the threat of French invasion.

Twentieth-century warfare and its targeting of civilian populations from the air exponentially raised the stakes. During World War I, stealth German Zeppelins dropped bombs on the British seaside towns of Great Yarmouth and King’s Lynn followed by London. Then in 1937, Nazi Germany tested its new air force, the Luftwaffe, with a three-hour onslaught of bombs and gunfire over the Basque town of Guernica in northern Spain_._

The British government established the Air Raid Precautions Committee in 1924. But it wasn’t until the formation of an Imperial Defence Committee sub-committee on evacuations in 1938 that discussions on evacuation measures really progressed, Starns explains.

The Anderson Committee, which was named for its head Sir John Anderson, began constructing and implementing plans for a wartime evacuation policy to relocate civilians from cities and other areas considered at high risk of being bombed (regarded as “evacuation” zones) to relatively safer locations, usually rural communities (known as “reception” zones).

Among those recommended for evacuation were school-age children, pregnant women, mothers with their young children (under the age of five), and teachers.

The report also made it compulsory for private householders in reception areas to take in evacuees, while civilian evacuation would be a voluntary process. In a BBC radio broadcast on the evening of January 6, 1939, Minister of Health Walter Elliott announced, “We want this to be a matter of real human relationship and affection, a willing host and a willing guest. The whole nation will have to feel itself as one if such a crisis really comes.”

The Women’s Voluntary Service (WVS) crisscrossed the country carrying out surveys on millions of homes, collecting information on the number of rooms and current inhabitants to ascertain how many private “billets” were available. Host families would be paid 10 shillings and sixpence for the first unaccompanied child, and 8 shillings and sixpence for any subsequent children.

Shortly after German forces goose stepped into Czechoslovakia on March 15, 1939, parents who wished for their children to be evacuated—and who did not have the means to arrange for it privately—were told to register at their local school. There were further government appeals, canvassing of doors, a leaflet outlining the evacuation program delivered to every home, and evacuation drills rehearsed in the schools.

“At any moment we expected to be ordered to assemble and prepare to march off to the railway station or to board a bus and lessons were practically at a standstill,” said Jean Noble in a testimony to social historian Gillian Mawson for Britain’s Wartime Evacuees.

Parents across the country were faced with an agonizing dilemma: send their children away to live with strangers in areas of “relative” safety, or stay in the city and face the threat of likely bombardment together.

For the children whose parents chose evacuation, “many remember leaving without an opportunity to say goodbye to their parents,” Mawson wrote. They would go off to school—with a gas mask and pre-stamped postcard bearing their parents’ name and address, among other recommended items—and not return home. Parents received news of their children’s destinations, and how they could be reached by mail, from notices posted on the school gate.

Additional waves of evacuation followed, with 1.25 million people fleeing cities during the Blitz in 1940. While most children journeyed by train within the British countryside, about 3,000 children were sponsored by the government to travel to more far-flung places, including Canada, Australia, and New Zealand. Many evacuees also returned early; by January 1940, around 900,000 evacuees had returned to the cities, despite calls by the Ministry of Health to “leave the children where they are.”

Overall, the uptake, considering most urban schools were shuttered, was lower than expected. “There is a myth that most children were evacuated to the reception areas for the duration of the war, when in fact, more children could be found living in the cities,” says Starns.

For those who were evacuated, experiences varied considerably. Some children were met with incredible kindness while others endured incredible cruelty, including physical and sexual abuse.

Upon visiting a home for children who had been evacuated, Labor peer Marjory Allen—Lady Allen of Hurtwood—was so horrified over their treatment that she wrote an impassioned letter to The Times on July 15, 1944. In response, the paper received more letters about deprived children than on any other single subject during the war.

The government was compelled to take action: The resulting Children Act of 1948 emerged as a landmark law transforming how vulnerable children were safeguarded by the State.