When war broke out in Europe a century ago, my mother’s mother, Gladys Gelperin, was a bright, plump, privileged 12-year-old girl living in the shtetl of Krasnoye north of Minsk. My grandmother grew to young womanhood over the next four years in the war zone known as the Eastern Front. Her beloved older brother was drafted into the czar’s army and never seen again.

Cossacks swept through Krasnoye, chasing Jews from their homes, plundering their shops and desecrating the synagogue. Gladys’s father, a merchant and former mayor of the town, died shortly after the war, devastated by the death of his son.

Though I saw my grandmother at least once a week from the time I was born until I went away to college, only once did she speak to me of the Great War that effectively ended her childhood. Just one story broke her silence on the subject.

In 1917, when the Kaiser’s army seized Krasnoye, my grandmother’s family’s house was requisitioned for the German commander. “The Russian soldiers were like animals—just terrible to the Jews,” my grandmother told me, practically spitting, six decades later. “But this German who lived with us was a perfect gentleman. He helped my mother hide our money and valuables. What he used, he paid for. He told my father that if we ever needed any help after the war, we should get in touch.”

My grandmother was proud, wise and self-confident—nobody’s fool—but there was a touch of dreaminess when she told me about how gentlemanly “her” German had been. I wondered if she had a crush on him. I wondered what this officer was doing 24 years later when the German army returned to Krasnoye—as murderers.

I dwell on this family story because I think it goes to the heart of the Jewish experience in the global conflict that began 100 years ago this summer. The Great War marked the critical divide between Jewish families in Europe and the United States—and in Eastern Europe it set the stage for the catastrophes of the war that followed. As Yale history professor Jay Winter, author of Remembering War: The Great War Between Memory and History in the 20th Century (Yale University Press), puts it, “The chaos that Jews suffered in Eastern Europe during the period of the war and beyond was radically different from the experience of American Jews. The U.S. suffered a bloody nose in the war, while in Eastern Europe the conflict left lasting scars politically and socially.”

Those scars were painful for Jews. Not only did Jewish conscripts fight on both sides—some 450,000 Jews served in the armies of Germany and Austro-Hungary, while the Allies had over one million Jewish men in uniform—but the battlefields of the Eastern Front sprawled over the ancient heartland of European Jewish life. Jewish soldiers and civilians alike became casualties of the war. The lootings, forced evacuations, accusations of spying and economic disaster that my grandmother’s family endured were the common experience of Jews from Konigsberg to Kosovo.

By comparison, American Jews were lucky—and they knew it. Not that their sons were spared fighting and dying in the trenches of France and Flanders. America’s Jewish community at the time was made up largely of East European immigrants, but that did not exempt young men from the draft that was reinstated after the United States declared war on Germany in April 1917. As long as an immigrant was between the ages of 21 and 30, was not an enemy alien and had taken out his first papers declaring his intention of becoming a citizen, he was eligible. Jews, the second largest immigrant group after Italians, were drafted in large numbers. Eventually, some 225,000 American Jews served, accounting for 5.73 percent of Americans in uniform, though they only made up 3.27 percent of the population.

Loyal service, however, was no safeguard against ethnic and religious slurs. As they streamed into makeshift training camps in the autumn of 1917, Jewish recruits discovered that anti-Semitism was rife in all branches of the United States military. Samuel Goldberg was 107 years old in the summer of 2006 when I interviewed him in Providence, Rhode Island. He vividly recalled being greeted with the words “Jesus Christ, a Jew in the cavalry!” when he showed up to train with the 12th Cavalry Regiment at Fort Oglethorpe in Georgia. Another conscript recalled the WASP commanding officer of the heavily Jewish 77th Division despaired that the Lower East Side tailors and peddlers who arrived for training at Long Island’s Camp Upton were “the worst possible material from which to make soldier-stuff.”

But in the crucible of combat, men forged bonds transcending their prejudices; and immigrant soldiers, perhaps Jews most of all, developed tremendous pride in serving their adoptive country. Len Epstein, a retired New Jersey schoolteacher, said his Russian Jewish immigrant father, Meyer, nearly starved trying to keep kosher while fighting with the Fourth Infantry Division in the Argonne Forest in France. Nevertheless, Private Epstein always spoke reverently about his military service. “To my father, Armistice Day was like Yom Kippur,” said his son.

Jessica Cooperman, an assistant professor of religion studies at Muhlenberg College in Allentown, Pennsylvania, who has been researching American Jewish service in World War I, concludes that “American Jews had a dual experience. On the one hand, military service heightened their sense of Jewishness and, on the other hand, it boosted their sense of entitlement to be treated like full-fledged Americans. In the diaries of Jewish doughboys [the informal term for infantrymen in World War I] that I’ve read, I see these two strands: ‘Now I know for sure I am American’ [and] ‘Now I want to rededicate myself to Judaism.’”

Cooperman feels that, on balance, military service was an Americanizing experience that pushed Jewish recruits and their families into the mainstream. Michael Neiberg, a professor of history at the United States Army War College in Carlisle, Pennsylvania, concurs. “A lot of Jewish families left Russia to avoid having their sons drafted into the Russian army,” he said, “but their service with the U.S. forces in France stamped them with Americanness in a way that was never true in the armies of Europe. It meant something to a Jew to serve in the U.S. Army.”

It meant something different for a Jew to serve in the German army during the Great War. Germany’s Jews, the most assimilated in Europe, rallied enthusiastically around the Kaiser, only to be accused of shirking and war profiteering. The notorious “Jew count,” a 1916 census of German Jewish military service, was commissioned by anti-Semitic elements in the German army who believed Jews were avoiding active combat. The survey proved the opposite: Eighty percent of Jewish conscripts were fighting at the front, which probably explains why the results were never published—an ominous sign of things to come.

Friedrich “Fritz” Adler was one of those assimilated German Jews who had volunteered to fight for the Kaiser. Steve Greenstein, the husband of Adler’s granddaughter, sent me a photo that Grandfather Fritz kept of Jewish comrades proudly wearing talitot draped over their German uniforms. “Unlike the Russian Jews who historically were conscripted into service,” Greenstein comments, “the German Jews felt a sense of loyalty to their homeland and an obligation to serve just like their non-Jewish neighbors.”

The bitter irony of the ensuing decades is not lost on Adler’s family. Said Greenstein, “The fact that fellow Jews were killed in mass executions by the same countrymen they fought for a mere 25 years earlier is something I’ve always had trouble getting my head around.”

Just as bitter was the legacy of the Great War encounter between the German army and the shtetl Jews of Eastern Europe. When the czar’s regime collapsed during the 1917 Russian Revolution, large parts of the former Pale of Settlement fell into German hands. War-weary Russian Jews like my grandmother’s family greeted the German army as liberators—or at least as the lesser of two evils.

But rosy family stories paper over a more complicated reality. The German occupiers, though less violently anti-Semitic than their Russian counterparts, viewed the Ostjuden (East European Jews) as dirty, poor, backward and disease-ridden—and that perception, according to Winter, “led to contempt, which in turn led to racial hatred.” The post-Great War spike in German anti-Semitism had many causes, but one factor was the German soldiers’ perception of the Ostjuden as “other”—i.e., subhuman.

As for the Ostjuden, the myth of the humane German left them tragically vulnerable to Nazi atrocities in the next war. “They assumed that the 1941 Germans would not be any different from those in 1918,” wrote one survivor of the yeshiva town of Volozhin (in present-day Belarus) about the reaction of his neighbors to the Nazi invasion. “They said, ‘It is not reasonable that this cultivated and organized nation could change during one generation.’”

The Soviet Union, which seized eastern Poland and present-day Lithuania at the start of the war under the terms of the Molotov-Ribbentrop pact, contributed to the tragedy by deliberately censoring reports of German anti-Semitism.

The German officer billeted in my grandmother’s house offered to help the family in any way he could, and three years after the armistice, the family took him up on it. In the spring of 1921, Gladys, now 18, left Krasnoye for America; the German officer met her at the Paris station and escorted her to her connection to Southampton, England. My grandmother arrived in New York onboard the Aquitania on April 9. She was lucky. Twenty years later, the Nazis chose Krasnoye as the site of a regional concentration camp for Jews who had survived the slaughters in surrounding shtetls, including Volozhin.

I was in college when my grandmother told me about the German officer. For years, I thought of it as one of those quaint family stories—colorful, trivial, a bit frayed in the retelling. Now I understand that this was part of a vast mosaic of family stories that stretches from New York to Berlin, Minsk and Moscow.

For my family, the chapter of history that began with the assassination of Archduke Franz Ferdinand 100 years ago this summer ended in the spring of 1943 when the Krasnoye concentration camp was liquidated. Before the Great War, my European and American relatives were one family, bound by love, food, worry, a sense of humor—in short, our Jewishness. But the chain of events set in motion by the Great War sundered these ties forever. First the war sent the two branches of my family down sharply different paths; then its aftermath reduced two branches to one.

David Laskin is the author of The Long Way Home: An American Journey from Ellis Island to the Great War (Harper Perennial) and The Family: Three Journeys into the Heart of the 20th Century (due out in Penguin paperback in September).