Mohammed Shah Kajar, ruler of Persia, was determined to possess the Afghan city of Herat; a city long known as the "Key to India." On June 23, 1837 he marched out of Tehran towards Herat, 650 miles to the east. After two earlier frustrated attempts to take the city, he did not intend to fail again.

Count Ivan Simonich, the Russian Minister to Persia, had encouraged the expedition from the beginning and had made promises of Russian aid. On the other hand, the British Minister to Persia, Sir John McNeill, had consistently pointed out the difficulties and had made clear the official British disapproval of the whole affair. At the last minute, the Shah had doubts of Russian support in case of real British opposition but Count Simonich was able to reassure him and he marched.

The events that unfolded during the next few years as a consequence of the Shah's decision determined the pattern of relations in this part of the world for the rest of the nineteenth century. The interests of Britain and Russia came into sharp conflict for the first time and limits were set to the power of each that were not to be exceeded. The affair also made clear that Persia, Afghanistan, and the other states of the area were no longer in control of their own destinies, but were becoming pawns in a power struggle between outside powers.