Jerusalem was the capital of Israel during biblical times. In modern times, the legitimacy of Israel's claim over Jerusalem as its capital is still disputed.
Roman legions razed Jerusalem in 70 AD and many Jews were killed or enslaved.
To Jews, it has always been the holy and traditional capital of their nation. To Christians, it is sacred because Jesus Christ conveyed his teachings and was crucified here. To Muslims, it must be protected because within the city is the Al-Aqsa Mosque, the third holiest site on Earth to them.