United Arabia

In 1916, during the Arab Revolt, Hussein bin Ali was declared King of the Arab Countries. However, he only actually ruled over Hejaz, as he was not given the lands promised to the Arabs by Britain, partially due to the Sykes-Picot. Also, his claim was contested by the Rashidis and the Saudis. However, what if the Sykes-Picot agreement never takes place and an independent Arab state is formed (which is still a puppet state of Britain, but gains full independence later).

1918: Creation of a Kingdom
In 1918, Arabia becomes independent from the Ottoman Empire, becoming the Kingdom of Arabia. King Hussein grudgingly agrees to Arabia being a puppet state of Britain, but secretly, he plans to revolt against the British when Arabia becomes strong enough (we'll get to that later). At this point it only consists of OTL Lebanon, Syria, Iraq, Palestine, Jordan, and Hejaz. After the Ottoman Empire is defeated in 1918, all of these countries become part of the Kingdom of Arabia. After this, Britain and Arabia set their eyes on Nejd and Jabal Shammar. Not wanting to seem like warmongers, they attempt to find a reason to go to war with the Saudis and Rashidis.

1919-1925: Three-way war
See also: Arab War of Unification