In 1943, Henk Zanoli took a dangerous train trip, slipping past Nazi guards and checkpoints to smuggle a Jewish boy from Amsterdam to the Dutch village of Eemnes. There, the Zanoli family, already under suspicion for resisting the Nazi occupation, hid the boy in their home for two years. The boy would be the only member of his family to survive the Holocaust.

Seventy-one years later, on July 20, an Israeli airstrike flattened a house in the Gaza Strip, killing six of Mr. Zanoli’s relatives by marriage. His grandniece, a Dutch diplomat, is married to a Palestinian economist, Ismail Ziadah, who lost three brothers, a sister-in-law, a nephew and his father’s first wife in the attack.

On Thursday, Mr. Zanoli, 91, whose father died in a Nazi camp, went to the Israeli Embassy in The Hague and returned a medal he received honoring him as one of the Righteous Among the Nations — non-Jews honored by Israel for saving Jews during the Holocaust. In an anguished letter to the Israeli ambassador to the Netherlands, he described the terrible price his family had paid for opposing Nazi tyranny.

“My sister lost her husband, who was executed in the dunes of The Hague for his involvement in the resistance,” he wrote. “My brother lost his Jewish fiancée who was deported, never to return.”

Mr. Zanoli continued, “Against this background, it is particularly shocking and tragic that today, four generations on, our family is faced with the murder of our kin in Gaza. Murder carried out by the State of Israel.”

His act crystallizes the moral debate over Israel’s military air and ground assault in the Gaza Strip, in which about 2,000 people, a majority of them civilians, have been killed. Israel says the strikes are aimed at Hamas militants who fire rockets at Israeli cities and have dug a secret network of tunnels into Israel...