Nadir Khan became a military general in Amanullah Khan's monarchy after the British planned it for him so he can suppress it to enhance his reputation. When the Anglo-Afghan war started in 1919, Nadir volunteered to lead the National Army to the south and to fight the enemy. When Nadir crossed the border, the British left every single fort on the other side of the durand line so Nadir could capture them. This made Nadir Minister of War after the war. Amanullah Khan became suspicious about Nadir’s incantations and Amanullah Khan understood his schemes and considered his staying unnecessary in Kabul. Nadir was appointed as the Ambassador of Afghanistan in Paris, and his brother Hashim was sent to Moscow as the cultural attaché. Both of the brothers considered the mentioned appointments as exile. Thus, after those appointments they strengthened their relations with the British. Eventually, Nadir before leaving Kabul met in person with the British Ambassador, Humphreys, at his office. During the mentioned meeting, which is written in the declassified document of the British, Nadir promised to follow whatever role would be given to him by the British. After this, Nadir, in Paris, proposed numerous plans to the British, to topple Amanullah Khan's government and the King of Afghanistan.
Shortly after a rebellion by Pashtun tribesmen and forces of Habibullah Kalakani began against the monarchy, Mohammad Nadir was exiled due to disagreements with King Amanullah. After the overthrow of Amanullah Khan's monarchy by Habibullah Kalakani, Mohammed Nadir returned to India and acquired military support from the British. He returned to Afghanistan with his British supported armies and took most of Afghanistan from Habibullah Kalakani. By October 13 of 1929, Mohammad Nadir Khan captured Kabul and subsequently sacked the city. Nadir Shah then asked for a truce with Habibullah Kalakani and asked him to join him so that they could discuss the political upheavals and come to a resolution. Nadir signed an oath on the Qur'an that he would honor his truce. Kalakani accepted Mohammed Nadir's truce and went to Nadir's meeting accompanied by Nadir's religious envoy. Despite Nadir's oath on the Qur'an and the religious envoy, upon his arrival Habibullah Kalakani was shot and hanged. Mohammad Nadir Khan then declared himself King, or Shah, of Afghanistan on October 16, 1929.
Nadir Shah named a ten-member cabinet, consisting mostly of members of his family, and in September 1930 he called into session a loya jirga of 286 which confirmed his accession to the throne. In 1931 the King promulgated a new constitution. Despite its appearance as a constitutional monarchy, the document officially instituted a Royal oligarchy, and popular participation was merely an illusion.
Although Nadir Shah placated religious factions with a constitutional emphasis on orthodox denominational principles, he also took steps to modernize Afghanistan in material ways, although far less obtrusively than Amanullah. He improved road construction, especially the Great North Road through the Hindu Kush, and methods of communication. He forged commercial links with the same foreign powers that Amanullah had established diplomatic relations with in the 1920s, and, under the leadership of several prominent entrepreneurs, he initiated a banking system and long-range economic planning. Although his efforts to improve the army did not bear fruit immediately, by the time of his death in 1933 Nadir Shah had created a 40,000-strong force from almost no national army at all.
He waged a large scale campaign under British influence against the non-Pashtun ethnic living in Afghanistan in attempt to continue the Pashtunization plan of his predecessor Abdur Rahman Khan. During his reign thousands of Afghan intellectuals were either imprisoned or killed. Many fled abroad, especially to the Soviet Union. The already-in-crisis press was heavily censored and power was distributed among his own relatives and family members.
During his reign, Nadir Shah had to suppress attempts to reinstate Amanullah Khan to the throne. His strategy in suppressing his opposition was to set ethnic groups against each other, mainly Pashtuns versus Tajiks. This led to the destruction of the Shamali plains north of Kabul.