From the late 17th century, the Omani Sultanate was a powerful empire, vying with Portugal and Britain for influence in the Persian Gulf and Indian Ocean. At its peak in the 19th century, Omani influence or control extended across the Strait of Hormuz to modern-day Iran and Pakistan, and as far south as Zanzibar (today part of Tanzania, also former capital). As its power declined in the 20th century, the sultanate came under the influence of the United Kingdom.
When Sultan Sa'id bin Sultan Al-Busaid died in 1856, his sons quarreled over the succession. As a result of this struggle, the empire—through the mediation of the British Government under the Canning Award—was divided in 1861 into two separate principalities: Zanzibar (with its African Great Lakes dependencies), and Muscat and Oman.
The Hajar Mountains, of which the Jebel Akhdar is a part, separate the country into two distinct regions: the interior, known as Oman, and the coastal area dominated by the capital, Muscat. In 1913, control of the country split. The interior was ruled by Ibadite imams and the coastal areas by the Sultan. Under the terms of the British-brokered Treaty of Seeb of 1920, the Sultan recognised the autonomy of the interior. The Sultan of Muscat would be responsible for the external affairs of Oman.
Muscat and Oman was the object of Franco-British rivalry throughout the 18th century. During the 19th century, Muscat and Oman and the United Kingdom concluded several treaties of friendship and commerce. In 1908 the British entered into an agreement of friendship. Their traditional association was confirmed in 1951 through a new treaty of friendship, commerce, and navigation by which the United Kingdom recognized the Sultanate of Muscat and Oman as a fully independent state.
The Sultan of Oman declared war on Germany on September 10, 1939.