He became a British subject in 1910, and in World War I he was (1916-19) director of the British Admiralty laboratories. While a lecturer at Manchester he became famous for discovering how to use bacterial fermentation to produce large quantities of desired substances. He is considered to be the father of industrial fermentation. He used the bacterium Clostridium acetobutylicum (the Weizmann organism) to produce acetone. Acetone was used in the manufacture of cordite explosive propellants critical to the Allied war effort (see Royal Navy Cordite Factory, Holton Heath). Weizmann transferred the rights to the manufacture of acetone to the Commercial Solvents Corporation in exchange for royalties.
In 1917, he worked with Arthur Balfour to obtain the milestone Balfour Declaration, stating that the British government "views with favour the establishment in Palestine of a national home for the Jewish people". A founder of so-called synthetic Zionism, Weizmann supported grass-roots colonization efforts as well as higher-level diplomatic activity. Siding with neither Labour Zionism on the left nor Revisionist Zionism on the right, Weizmann was generally associated with the centrist General Zionists. In the 1917, expressed his view of Zionism in the following words,
On January 3, 1919, he and the future King Faisal I of Iraq signed the Faisal-Weizmann Agreement establishing relations between Arabs and Jews in the Middle East. After 1920, he assumed leadership in the world Zionist movement, serving twice (1920-31, 1935-46) as president of the World Zionist Organization. In 1921, Weizmann went along with Albert Einstein for a fund-raiser to establish the Hebrew University in Jerusalem.
Concurrently, Weizmann devoted himself to the establishment of a scientific institute for basic research in the vicinity of his sprawling estate, in the town of Rehovot. Weizmann saw great promise in science as a means to bring peace and prosperity to the area. As stated in his own words :
"I trust and feel sure in my heart that science will bring to this land both peace and a renewal of its youth, creating here the springs of a new spiritual and material life. [...] I speak of both science for its own sake and science as a means to an end."His efforts led in 1934 to the creation of the Daniel Sieff Research Institute, that was financially supported by an endowment by the Baron Israel Sieff in memory of his late son. Weizmann actively conducted research in the laboratories of this institute, primarily in the field of organic chemistry. In 1949 the Sieff Institute was renamed the Weizmann Institute of Science in his honor. Weizmann's success as a scientist and the success of the Institute he founded make him an iconic figure in the heritage of the Israeli scientific community today.
In 1936 he addressed the Peel Commission, set up by Stanley Baldwin, whose job it was to consider the working of the British Mandate of Palestine. The Commission published a report that, for the first time, recommended partition, but the proposal was declared unworkable and formally rejected by the government.
During World War II, he was an honorary adviser to the British Ministry of Supply and did research on synthetic rubber and high-octane gasoline. (Formerly Allied-controlled sources of rubber were largely inaccessible owing to Japanese occupation during World War II, giving rise to heightened interest in such innovations). Tragedy struck when his younger son Flight Lt Michael Oser Weizmann, serving as a pilot in the British Royal Air Force, was killed when his plane was shot down over the Bay of Biscay.
Shmuel Katz, historian and former Etzel member, writes that in his opinion Weizmann ignored the fact that his son was killed as a fighter for the freedom of his children, "but Moyne paid with his life for his direct responsibility for the deaths of innocent men, women and children that their only crime was belonging to the same people of Weizmann". Katz also repeats that Moyne avidly objected the notion of Zionism, that he revealed total indifference to the fate of the Jews that could have been saved, and he repeats Moyne's statement of "what should I do with a million Jews?".
He met with United States President Harry Truman and worked to obtain the support of the United States for the establishment of the State of Israel. Weizmann became the first President of Israel in 1949. His nephew Ezer Weizman also became president of Israel. He is buried beside his wife, Vera, on the Weizmann estate, which is located on the grounds of Israel's premier science research institute, The Weizmann Institute of Science.