On the eve of the War of Independence and during its first 6 months (December 1947- May 1948), Hagana forces served as the Yishuv's and the State of Israel's informal army. It operated in both rural and urban settlements. Hagana fighters defended the newly born Jewish state and saved it from the horrible fate planned by the Palestinians and the Arab countries.
In no time the12 Hagana brigades (a total of 42 battalions) turned to be the land forces of the new Israeli army. Also, the Hagana forces formed the Air Force, Navy, Armored brigades, Artillery and Engineering corps, Military Intelligence, and Military Logistics units - all transformed from Hagana to the IDF. Hagana Purchased and shipped to Israel vital weapons (including aircraft, ships, artillery and armored vehicles). Other weapons were manufactured in Hagana's military industries (TAAS).
At the beginning of the war the Hagana's fighting forces included some 4,000 enlisted men and women, growing constantly during the war to about 40,000. In addition, during the first year of the war 25,000 overseas volunteers were organized and trained by Hagana delegates around the world (20,000 by Gachal and 5,000 by Machal) and joined the Israeli forces.
On May 31st, 1948, two and a half weeks after the departure of the British and the Declaration of Independence, the Hagana was declared formally IDF and sworn in. The Order of the Day, declaring the establishment of IDF read: "The Hagana has become the regular army of the State of Israel".
The General Staff of the Hagana became the General Staff of IDF, and its Chief of Staff, Ya'acov Dori, became the first Chief of Staff of IDF. The commanders of Hagana became the commanders of the IDF, demonstrating the same military capabilities, war experience and pioneering spirit of the Hagana in fighting and defeating the invading Arab armies.