The events of September 1969, proved to be pivotal both for Crown Prince Hasan and for Libya. As Crown Prince, Hasan was first in line succeed to the Libyan throne. The ailing Idris presented a signed document on 4 August 1969 to the President of the Libyan Senate, whereby Idris was to abdicate in favour of the Crown Prince. The instrument of abdication specified 2 September, the date when the King undertook formally to stand down. Indeed, the Crown Prince was already exercising regal powers in the name of King Idris, in the run up to 2 September. However, on 1 September, while Idris was out of the country undergoing medical treatment, a group of Libyan army officers, among them Colonel Muammar al-Gaddafi, staged a rebellion and announced that King Idris was deposed. Since King Idris I was unable to complete his reign as envisaged by his instrument of abdication Hasan never actually became king. His legacy, however, was in his role of Crown Prince, exercised between 1956 and 1969, towards the end of which he was the de facto ruler of Libya. As Crown Prince, he repeatedly undertook official trips abroad, notably to negotiate the purchase of U.S.-built jet fighters from the Kennedy Administration for the Libyan Air Force.