Why are the following states independent and not a part of Saudi Arabia: Qatar, UAE, Oman, Yemen, and Kuwait?
While their annexation might be difficult in our current time period, it should have been much less politically dangerous during the Saudi Unification wars.
Under the terms of The Treaty of Darin with the United Kingdom, signed Dec. 26, 1915 in exchange for recognition of sovereignty, ibn Saud contracted to not attack Kuwait, Qatar, and the Trucial States (Oman).
Yemen has traditionally not been claimed by the House of Saud for geographical reasons, including physical separation, distinct terrain, and the existence of a natural border in the form of the separating mountain range.
Kuwait, Qatar, Bahrain, the Trucial States (modern UAE) and Oman were all protected by treaties with Great Britain. Yemen proper was never claimed for the reasons Pieter explained above.
That said, Ibn Saud did manage to take away about 2/3rds of the territory claimed by Kuwait, and this was recognized by Great Britain as part of the Uqayr Protocols (1922). Also, Ibn Saud did conquer the regions of Assir and Jizan (southwestern corner of modern Saudi Arabia) which had independent local rulers but were claimed by Yemen, and a war was fought between the two countries before Yemen recognized Ibn Saud's rule over those areas.
