
The land that is now Israel/Palestine was under Ottoman rule until WW1 when it became a British mandate called Palestine. Increasing Jewish emigration to Palestine was encouraged by state antisemitism in Europe and the creation of the Zionist Organization. Both the Jews and the Arabs there launched revolts against British rule, and against each other, and the Brits left
The UN came up with a partition plan to parcel out equal land to Jews and Arabs with Jerusalem as an international zone, the Jewish side accepted this, the Arab side did not. A war broke out and the surrounding Arab countries all intervened looking to get a slice for their own. The Palestinian Arabs were caught in the middle, and ejected from land they owned when the Jewish forces won the war. This is called the Nakba (catastrophe)
The Palestinians settled behind the 1949 armistice line, and the state of Israel was created occupying more land than the UN had apportioned, due to winning the war. The West Bank was annexed by Jordan, and the Gaza Strip by Egypt. In 1967 the Israelis launched a preemptive war against Egypt Syria and Jordan, occupying the Sinai, the Gaza Strip, and the West Bank. They gave back the Sinai, but Gaza and West Bank remain occupied by the military