During the Japanese occupation of Korea (1910-1945), he was elected as the president of the provisional government. After Korea was liberated, he became the first president (August 1948 - April 1960, four consecutive terms) of South Korea during the Korean war.
Rhee became greatly unpopular with his allies when he refused to agree to a number of ceasefire proposals that would leave Korea divided. His hope throughout the war was that with UN help he would be made leader of a united Korean peninsula, and tried to veto any peace plan that would not eliminate the north completely. He pushed for stronger methods to be used against the People's Republic of China and was often irate at the US reluctance to bomb Mainland China.
His rule was autocratic and he often oppressed dissent and ruthlessly treated captured communists.
He was the co-author of the Petition from the Koreans of Hawaii to President Theodore Roosevelt, along with P.K. Yoon, which appealed to the Americans in face of Japanese aggression in 1905.