First Anglo-Afghan War
One of totaling three major wars between Great Britain and Afghanistan. The initial two of them were fought in the 19th century (1839-1842; 1878-1880), the final one in the twentieth century (1919), only one year after the end of World War I.
The First Anglo-Afghan War was the first major war during the so called "Great Game", which was the competition for power and influence in Central Asia between the United Kingdom and Russia in the 19th century.
The principal cause of the war was Britain's fear of Russian to invade India (the Russian Empire had expanded towards the British part of India). Consequently, the British invaded Afghanistan, a neighboring country of India in order to form an alliance with Afghanistan against Russia. Dost Mohammad, who was Afghanistan's King at that time agreed to the alliance but demanded something in return, namely getting Peshawar back, an Afghan province they had lost to India. When Britain refused to help Dost Mohammad started negotiating with Russia, which made Lord Auckland, India's Governor-General assume Afghanistan was against Britain. However, when the Afghan-Russian negotiations failed, Russia formed an alliance with Persia since both powers were interested in getting parts of Afghanistan for different reasons.
Lord Auckland then intended to appoint a new, pro-British ruler in Afghanistan to replace Mohammad.
In order to vindicate this intention he issued the so-called Simla Manifesto (1838), which stated the alleged arguments for Britain to intervene in Afghanistan. The prime argument of the manifesto was the supposed fact that India needed to have a confidable ally on India's western frontier.
Sources:
Barfield, Thomas. Afghanistan: A Cultural and Political History New Jersey, Princeton University Press. [2010].
David, Saul. Victoria's Wars London Penguin Books, 2007.