The Arch of Titus is to commemorate the Roman victory after 42 months of the First Jewish-Roman war
The war was triggered by Judeans who saw the decay in Rome under Nero as a perfect chance to get their independence ('What have the Romans ever done for us...?') in AD 66
This lasted 42 months as foreseen in the book of Revelations and it ended in the utter, total destruction of Jerusalem, as foreseen in the same book
Josephus describes how the people in Jerusalem resorted to cannibalism in the end - parents killing their children for food - again foreseen in the book of Revelations
And Josephus also describes -- what seems INSANE to us - how, while Jerusalem was under siege, the Zealots started a civil war
Can you imagine that? You're under siege and you decide 'let's start a civil war'?!?
The Zealots attacked the Sadducees, pharisees etc
The Christians ran away - to the hills - as Jesus told them in Matthew 24 (the Olivet discourse) and not a single Jesus-movement (Christian) was killed
When the city finally fell, Josephus describes how the Roman soldiers didn't listen to their commanders, but seemed possessed in destroying the temple -- the Zealots were committing suicide attacks and had already killed their women and children - and the temple was razed to the ground and the temple menorah etc. were carried away. And, in the holy of holies, was placed an idol to the genius of the Princep - an abomination in the Holy of Holies - again as foreseen
Finally, the city was razed to the ground so thoroughly that the only bit remaining is the Wailing wall which was actually the wall of the FOUNDATION, not even of the temple. What we see as ancient Jerusalem is actually ancient Aelia Capitolina, a Roman city erected by Hadrian 60 years later (and which triggered off the Kitos war with Judeans massacring gentile Cypriots and Cyrenians)