The United States became the first country to recognize the new state of Israel only a few hours after it had declared its independence at the end of the British Mandate. The American recognition was crucial in Israel receiving the recognition of the rest of the world.
With the end of World War II and the discovery of the horrors that had befallen European Jewry, the United States supported some change in the status quo in Palestine, which was a British mandate. Initially, the United States pressured the British into allowing 100,000 Jewish refugees into Palestine. When the British turned over the case of Palestine to the United Nations, a special United Nations Commission was formed. The commission recommended the division of Palestine into separate Jewish and Arab states, and the United States actively supported this recommendation. On November 30th, 1947, the UN voted to approve the partition of Palestine into two states -- Jewish and Arab. The Jewish community of Palestine accepted this proposal, while Arabs of Palestine, as well as those of the surrounding states, rejected it.
In the months preceding the end of the mandate, civil war broke out between the Jews and the Arabs of Palestine. With the mandate coming to an end, the surrounding Arab states were expected to attack, and, by most observers, to emerge victoriously. The state department recommended that the United States support some form of UN stewardship. The Jews of Palestine, however, had other plans, and declared, on the afternoon of May 14th, national independence.
President Truman decided to give diplomatic recognition to the new state. While the US representative to the UN was informing the world that the US supported trusteeship, the American government was simultaneously granting official recognition to Israel. Israel went on to win the War of Independence.