Steven Spielberg’s 1998 film Saving Private Ryan may include some of the most horrific fighting scenes ever produced on film. But that isn’t its only element of realism. The film draws on the story of an actual soldier named Fritz Niland and a U.S. War Department directive designed to keep families from losing every one of their sons.

The film tells the story of Captain John H. Miller (played by Tom Hanks), who leads a platoon of GIs during the D-Day invasion of Normandy near the end of World War II. Their initial mission, along with nearly 175,000 other Allied service men, is to liberate France and defeat the Nazis.

After getting themselves off Omaha Beach (yes, those horrific fighting scenes), Miller is able to push his platoon up over the ridge and into the French countryside.

Just a few days into the invasion, Captain Miller receives revised orders from high command. His new mission: locate and rescue Private First Class James F. Ryan (played by Matt Damon), whose three brothers had been killed in the war within a few months of each other. Private Ryan was what was known as a “sole-surviving son” and the War Department wanted him back with his mother.

The problem was, the U.S. Army didn’t know exactly where he was. In the film, Captain Miller and his platoon search for the wayward soldier in house-to-house fighting. Eventually, they find Private Ryan and send him back home—but at great cost to Captain Miller and his men.

The real-life story that inspired Spielberg’s film began more than two years before the rescue depicted in Saving Private Ryan.

On the morning of November 13, 1942, Japanese torpedoes sank the American cruiser USS Juneau during the battle of Guadalcanal in the South Pacific. Onboard were five brothers: George, Francis, Joseph, Madison and Albert Sullivan, who had all enlisted into the U.S. Navy after the death of a friend at Pearl Harbor.

The Navy agreed to their request that all five would serve together on the same ship. It wasn’t a common practice by the U.S. military to place siblings together, but it wasn’t discouraged either. Some officials saw it as a way to keep family morale high. In fact, at least 30 sets of brothers were serving on the Juneau when it sank.

In response to the deaths of the Sullivan brothers—and several other sets of brothers who had perished up to this point in the war—the U.S. War Department realized it had to act. The result: its 1942 “sole-survivor policy,” later known as Directive 1315.15 Special Separation Policies for Survivorship.

Sometimes referred to as the “Only Son” or “Sole-Surviving Son” policy, the directive was designed to protect lone remaining family members from military duty. It was this directive that prompted the rescue of Sargent Frederick “Fritz” Niland in 1944, one of four brothers who served in the U.S. military during World War II.

Frederick Niland’s story provided direct inspiration for Saving Private Ryan and its title character of James Francis Ryan.

Before the U.S. entered World War II, brothers Preston and Robert Niland enlisted in the service. Edward and Fritz volunteered in November, 1942. Because of the War Department’s sole-survivor policy prohibiting siblings from serving together, the four brothers served in separate units. Edward served as a pilot in the U.S. Army Force in the Pacific; Robert landed in the 82nd Airborne Division: Preston served with the 4th Infantry Division; and Frederick was a member of the 101st Airborne Division, 501st Regiment. The three brothers were stationed in England awaiting the invasion of Europe.

Tragedy came in waves for the boys’ parents, Michael and Augusta Niland.

In May 1944, they received the news that their son Edward had been shot down over Burma and was missing. On June 6, 1944, Robert died on D-Day, and Preston succumbed the next day near Omaha Beach. Frederick had parachuted into Normandy and was temporarily separated from his unit.

When the Army heard of the deaths of the three boys, it determined to spare the Niland family the loss of their last child. A chaplain on the 501st Regiment, Fr. Francis Sampson, found Fritz and put in the paperwork to send him home.

Fritz was shipped back to England and eventually the United States to serve as a MP for the rest of the war. Happily, the Niland family later learned that their son Edward had survived his capture in a Burmese POW camp and was sent home before the war ended.

Since it was enacted during World War II, the sole-survivor policy has evolved. Passed by Congress in 1948 into law, the directive exempted the lone remaining son where one or more sons or daughters had died as a result of military service. However, the policy didn’t exempt surviving sons from registering for military duty. If there was a draft, these men could be called up, but would receive a deferment.

In 1964, Congress changed the law to exempt the sole-surviving son of a family where the father, or one or more sons or daughters died as a result of military service. The provision was changed to apply only during peace time and not during times of war or national emergency declared by Congress. The provision was also made voluntary. A soldier wishing to be sent home had to request the policy be applied.

In 1971, Congress amended the law to include not only the sole surviving son or daughter but also any son or daughter who had a combat-related death in the family. Since then, each U.S. military branch has established its own policy regarding separating family members in service.

Saving Private Ryan represents the military “sole-survivor” policy in simple terms: When a family experiences the loss of its sons due to a war, the lone remaining member will not have to serve.

The individual that inspired the Private James Francis Ryan character, Frederick Niland, wasn’t ever lost and no search party was sent out to find him. But Spielberg powerfully portrayed how the death of siblings in war—and the trauma their families experience—had become a burden the nation decided was too cruel to bear.