Operation Cast Thy Bread was a top-secret biological warfare operation conducted by the Haganah and later the Israel Defense Forces (IDF) beginning in April 1948 during the 1948 Palestine war. 

The operation involved the deliberate contamination of drinking water wells with typhoid bacteria, violating the 1925 Geneva Protocol. Its primary objectives were to frighten Palestinian Arabs and prevent them from returning to villages captured by the Yishuv, as well as to make conditions difficult for Arab armies attempting to retake territories. The operation resulted in severe illness among local Palestinian citizens.

Evidence from British, Arab, and Red Cross documents confirms that Zionist forces introduced poison into wells in Acre and Eilabun in Galilee, leading to a typhoid epidemic and “a state of extreme distress” among the inhabitants of Acre, which had been allocated to a future Arab state under the United Nations Partition Plan. The contamination of Acre’s aqueduct played a significant role in the rapid fall of the city to Haganah forces in May 1948. The Haganah also poisoned the depopulated Palestinian Arab village of Bayt Mahsir and water sources in Palestinian neighborhoods in Jerusalem.

The operation was initially carried out by ordinary IDF soldiers, but later entrusted to the Mista’arvim, an undercover unit that specialized in sabotage operations within enemy territory and disguised themselves as Palestinians. The biological agents were produced by a unit within the army’s Science Corps, known as HEMED BEIT, which was established in February 1948 under the direction of Yigael Yadin. The operation was authorized and overseen by Israeli Prime Minister David Ben-Gurion and IDF Chief of General Staff Yigael Yadin.

In May 1948, during Operation Shalach, four Israeli Special Forces soldiers disguised as Arabs attempted to poison the water supply in Gaza to hinder the advance of the Egyptian army; they were captured and executed by an Egyptian military court. Plans were also made to expand the campaign into neighboring Arab states such as Egypt, Lebanon, and Syria, but these were not carried out. Despite its use, the operation did not achieve the crippling effects its advocates had hoped for and was discontinued by December 1948.

Historians Benny Morris and Benjamin Z. Kedar have uncovered official Israeli military documentation that confirms the operation, providing new details about its scope, organization, and authorization. The operation was codenamed “Cast Thy Bread” and was part of a broader strategy during the Nakba, the forced expulsion and ethnic cleansing of approximately 800,000 Palestinians from their homeland. While Israel has vehemently denied the allegations, denouncing them as “wicked libel” and accusing Arab states of antisemitic incitement, the newly unearthed documents have cast doubt on these denials.

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