Arthur Conolly, Victorian Spy

In September 1840, intelligence officer Captain Arthur Conolly set out from Kabul for Central Asia. His orders were to persuade its rulers to unite to resist Russian advances and open their markets to British goods. Conolly, a devout Christian and abolitionist, had a higher agenda: to free thousands of slaves.

Writing to a fellow officer, he described his mission as part of a “ great game.” Quoted by Rudyard Kipling in his novel Kim, historians would use the phrase to describe the geopolitic...