Final answer:
The nation that referred to its leader as the "Shah" was Iran, where Reza Shah took power in 1925. This title was indicative of monarchical rule in Iran, a tradition that continued until the revolution of 1979.
Step-by-step explanation:
The Middle Eastern nation that used the term "Shah" for its leader, with Reza Shah having come to power in 1925, is Iran. The term "Shah" indicates a monarch and is equivalent to the title of a king. Iran has a rich history tied to the ancient Persian Empire, and the use of the term 'Shah' continued until the Iranian Revolution in 1979, when the last Shah, Mohammad Reza Pahlavi, was overthrown.
Various periods in Iran's history, like the rule of Shah Abbas during the Safavid Empire, highlight the longstanding tradition of referring to its ruler as the Shah. The legacy of the Shah was not limited to Iran's borders but also extended through cultural and political influences, to countries like Turkey, Syria, and even beyond.
Despite similar historical connectivity, neighboring countries such as Turkey, which was transformed under Ataturk's leadership post-World War I into a secular republic, and Syria, which gained independence from French Mandate in 1946 and went through various political transformations, did not use the term 'Shah'. Instead, they followed different trajectories in terms of their political leadership titles and structures.