Short answers, pulled from the story.
Sectarian conflict in the region began by 1920, following the dissolution of the Ottoman Empire and the partition of Ottoman Syria. The conflict escalated into civil war in 1947 after the UN adopted the Partition Plan for Palestine, and became an international war on the 14th of May 1948 with Israel's declaration of independence and the subsequent invasion by Egyptian, Lebanese, Syrian, Jordanian, and Iraqi armies.
The Balfour Declaration was a 1917 statement by the British government expressing support for the establishment in Palestine of a national home for the Jewish people, while specifying that nothing should be done to prejudice the civil and religious rights of existing non-Jewish communities. It was partly motivated by the belief of Prime Minister David Lloyd George that Jewish support was essential to winning World War I. Arabs regarded it as conflicting with British promises of an independent Arab state made to Husayn ibn Ali during the same war.
Before and during the 1948 Arab-Israeli War, 713,000 Palestinian Arabs were expelled or fled, becoming refugees. Historian Benny Morris argued that the decisive cause of their departure was predominantly the actions of Jewish forces, including expulsions, military assaults on residential areas, and fear of fighting, while Arab leadership orders were decisive in only 6 of 392 villages.
The Oslo Accords were signed in September 1993 following secret peace talks in Oslo, Norway. Israel recognized the PLO as the legitimate representative of the Palestinian people, while the PLO recognized Israel's right to exist, renounced terrorism and violence, and abandoned its stated mission to destroy Israel. Oslo II followed in 1995.
A report by the Strategic Foresight Group estimated the opportunity cost of conflict for the Middle East from 1991 to 2010 at 12 trillion dollars. Israel's share was almost one trillion dollars; Iraq's was approximately 2.2 trillion dollars; Saudi Arabia's was approximately 4.5 trillion dollars. The report calculated that had peace prevailed from 1991, the average Israeli citizen would have been earning over 44,000 dollars annually in 2010 instead of 23,000 dollars.
The Abraham Accords were diplomatic agreements that further calmed relations between Israel and several Arab states, reached by 2020. They built on decades of earlier diplomatic and economic accords that followed successive wars, including the Israel-Egypt peace treaty of 1979 and the Israel-Jordan peace agreement signed on the 26th of October 1994.