Task: Question Understanding
Input (English): Passage: In late 1920, the Persian Soviet Socialist Republic in Rasht was preparing to march on Tehran with "a guerrilla force of 1,500 Jangalis, Kurds, Armenians and Azerbaijanis", reinforced by the Bolsheviks' Red Army. This fact, along with various other disorders, mutinies and unrest in the country created "an acute political crisis in the capital." By 1921, the ruling Qajar dynasty of Persia had become corrupt and inefficient. The oil-rich nation was somewhat reliant on the nations of Britain and Russia for military and economic support. Civil wars earlier in the decade had threatened the government, and the only regular military force at the time was the Cossack Brigade. The Qajar shah in 1921 was Ahmad, who had been crowned at the age of eleven. He was considered to be a weak, incompetent ruler, especially after British, Russian and Ottoman occupations of Persia during World War I. In 1911, when the capital city, Tehran, had been seized by the Russians, armed Bakhtiaris tribemen, rather than Iranian regular troops, expelled the invaders. This further diminished the government's reputation, rendering it almost powerless in time of war. Britain, which played a major role in Persia, was dismayed by the Qajar government's inability to rule efficiently. This inefficiency was the background of a power struggle between Britain and Russia, each nation hoping to control Persia. On 14 January 1921, the British General Ironside chose to promote Reza Khan, who had been leading the Tabriz battalion, to lead the entire brigade. About a month later, under British direction, Reza Khan's 3,000-4,000 strong detachment of the Cossack Brigade reached Tehran.
Question: When approximately did Reza Khan's 3,000-4,000 strong detachment of Cossack Brigade reach Tehran?
Output (English): date