Is the Great Malhama a reality in Islam, and will the Mahdi emerge before it, and will Constantinople be conquered again?

The emergence of the Romans to fight the Muslims in Sham (Syria) is considered one of the major signs of the Hour. There will be a truce with the Romans, then they will betray it and come to fight the Muslims in a great battle (Malhama). After this Malhama, Constantinople will be conquered without a fight, and then the Dajjal (Antichrist) will appear. Jesus, son of Mary, will descend after the Malhama, the conquest of Constantinople, and the appearance of the Dajjal, and the Mahdi will be manifest at that time.