Duryodhana thought that the might of his armed forces was immeasurable, being specifically protected by the most experienced general, Grandfather Bhisma. On the other hand, the forces of the Pandavas were limited, being protected by a less experienced general, Bhima. He used diplomatic ways with his grandfather as well since he clearly felt that the victory of the Kurus depended on the presence of Bhismadeva.
He was also confident of his victory on account of the combined strength of all his friends. They were all determined to lay down their lives for Duryodhana’s sake. Indirectly, it was already concluded that all of them under Kauravas would die in the Battle of Kuruksetra for joining the party of the sinful Duryodhana.
Duryodhana did not know that defeat and death does not depend on military strength or bodily strength. When death will come, nobody can check. Death is God’s act. When Lord Krishna desires that "This man should be killed now," or "He must die now," nobody can check. If Lord Krishna desires to kill somebody, nobody can give him protection, no power can save him. And if He wants to save somebody, nobody can kill him. This is Lord Krishna's protection. This realization that only Lord Krishna decides and saves is the beginning of surrender.