It’s 100 years today since the conclusion of the San Remo conference, which gave birth to the maps of the Levant and Iraq which ultimately survive to this day - the mandate for Syria (including Lebanon) going to the French, and those for Palestine (and by consequence Transjordan) and Iraq to the British. There have been three major attempts* to undo these maps - the union of Egypt and Syria from 1958-61, the Iraqi annexation of Kuwait in 1990-91, and the ISIS campaign of 2014-2018, but their endurance is remarkable given the lack of legitimacy associated with them.
Some immediate consequences were a brief Franco-Arab war over Syria which ended with the French occupation of Damascus and the end of the short-lived Hashimite Kingdom of Syria, a major anti-British revolt across Iraq during the summer of 1920 which ended with the placing of the Hashimites on the throne of Iraq, and the Treaty of Sèvres of the following August which severely limited the emerging post-Ottoman Turkish state by granting territories in Anatolia and Thrace to the Armenians, Kurds, and Greeks. Ataturk would reverse these promises in the wars which followed.
The major long-term consequence, apart from the actual birth of these states, was the formal international recognition of the terms of the Balfour Declaration by which the mandate of Palestine was to be reserved for the development of the Jewish national home.
* (a more minor successful change was made with the Turkish annexation of the Syrian province of Alexandretta/Iskenderun, modern-day Hatay, in 1939.)