Anglo-Russian Entente
Upon his father's death, he assumed the title of Moshir al Dowleh, playing an important role in drafting the Persian Constitution of 1906. Hassan was given an Honorary Knight Grand Cross of the Order of St Michael and St George by the British crown in 1907. From 1907 to 1908, Pirnia was the Minister of Foreign Affairs, during which time he declared the Anglo-Russian Entente, which would divide Iran into zones of imperial influence, as null and void. He later became Minister of Justice before becoming Prime Minister for the first time in 1915. He would re-assume the office in 1920, and again in 1922 and 1923. One of Pirnia's key actions during his time as Prime Minister saw him prevent the introduction of the Anglo-Persian Agreement of 1919.