Short answers, pulled from the story.
The Emirate of Afghanistan emerged from the Durrani Empire after the Barakzai dynasty prevailed in Kabul. It is dated from 1823, when Afghan forces lost the Valley of Peshawar at the Battle of Nowshera on the 14th of March 1823. Before 1834, it was known as the Principality of Kabul.
The Treaty of Gandamak was signed on the 26th of May 1879 by Emir Mohammad Yaqub Khan and the British Empire during the Second Anglo-Afghan War. It gave Britain control of Afghanistan's foreign affairs and ceded Afghan territories in present-day Pakistan, in exchange for a subsidy and a British military withdrawal.
Emir Amanullah Khan signed the Anglo-Afghan Treaty of 1919 following the Third Anglo-Afghan War, which lasted three months. The treaty restored Afghanistan's full right to conduct its own foreign affairs and ended its status as a de-jure British protectorate.
The Durand Line is a border imposed on Afghanistan by the British in 1893. It cuts through historic Pashtun settlement regions and defines the boundary between present-day Afghanistan and Pakistan. The emirate was forced to accept it under British pressure during the reign of Abdur Rahman Khan.
The principal emirs included Dost Mohammad Khan, who founded the emirate and died in 1863; Sher Ali Khan, who signed no treaty but fled before dying in Mazar-i-Sharif in 1879; Abdur Rahman Khan, who suppressed uprisings and accepted the Durand Line; Habibullah Khan II, who maintained neutrality in World War One; and Amanullah Khan, who ended the emirate in 1926 by founding the Kingdom of Afghanistan.
Early flags were triangular and featured red and green colors with Quranic inscriptions and the names of the four Islamic caliphs in white. The first use of a coat of arms on a flag came under Abdur Rahman Khan, who used a solid black banner with a white emblem at the center. That emblem became the origin of later Afghan state insignia.