 Complete the following phrase:  When in 1451 the bankrupt Byzantines asked Mehmed to double the tribute for holding an Ottoman pretender for the throne, he used the request as a pretext for annulling all treaties with the Byzantine Empire. Nevertheless, when he proposed in 1452 to siege Constantinople most of the divan, and especially the Grand Vizier, Çandarlı Halil Pasha, was against it and criticized the Sultan for being too rash and overconfident in his abilities. On April 15, 1452, Mehmed ordered preparations to be made for the 