At the beginning of September 1914, Otto Hersing, Commanding Officer of "U-21", ventured to the Firth of Forth, home to the major British naval base at Rosyth. Hersing is known to have penetrated the Firth of Forth as far as the Carlingnose Battery beneath the Forth Bridge. At one point the periscope was spotted and the battery opened fire but without success. Overnight Hersing withdrew from the Forth, patrolling the coast from the Isle of May southwards. On the morning of 5 September, he observed HMS "Pathfinder" on a SSE course, followed by elements of the 8th Destroyer Flotilla. At midday, the destroyers altered course back towards the Isle of May while "Pathfinder" continued her patrol. Shortly thereafter, Hersing spotted "Pathfinder" on her return journey through his periscope and resolved to make an attack.