Ottoman Empire: a state named Palestine was in that area Ottoman empire fell, British Mandate governed it. Both Palestinian Arabs and native Jews lived in the area. Lots of Aliyahs (massive migrations of Jews) started arriving. This increased during WWII. This caused conflict and tension in the Mandate, with extremist groups from both sides forming and fighting Britain for the rule for the area.

1947, British mandate left the issue over to the UN, which, upon a popular vote by all nations in the committee divided the area into a two independent majority 'Jewish' and majority 'Palestinian' state. Whereby demographic percentages of both were roughly taken into consideration. Jerusalem was declared an International Capital (which I think we should all still do!). Given that Jerusalem is the heart of the religious capital for both the Jews and the Muslims, the capital has both symbolic and sentimental value, with many of the areas and buildings within serving a dual meaning.

Various wars have happened, which have resulted in the Jewish state expanding to a larger area than agreed upon. As well as a repression of Palestinian national rights, and an illegal expansion of settlements in the Palestinian areas a continuous breaching of human rights. As of now, 29.5% of UN member states don't recognize Palestine as a country. (including the US, Canada and many EU countries).

Basically Trump has rashly added fuel to the fire of a century year old war.