The British wish for Afghanistan to remain…
November 1856 CE
In the context of the Great Game, they oppose an extension of Persian influence in Afghanistan because of the perception that Persia is unduly influenced by the Russians.
The Persian influence on Central Asia had caused the creation of Greater Iran, and the Persians had succeeded in taking Herat on October 25.
In response, the British Governor-General in India, acting on orders from London, declares war on November 1.
Separate from and preceding the dispute over Herat is an incident concerning one Meerza Hashem Khan, whom the British ambassador had hoped to appoint as a secretary in the mission in Tehran.
The Persians had objected, creating a dispute that escalated when rumors appeared that the British ambassador had improper relations with the man's wife, who was the sister of the Shah's principal wife.
The dispute had escalated still further when the Persians arrested the woman; the British ambassador had broken relations when they refused to release her.
Indeed, the initial mobilization of British forces begins in response to this incident, although it is unlikely that the British would have gone beyond the occupation of one or two islands in the Persian Gulf had the issue of Herat not arisen.