At the outbreak of WWI the German light cruiser SMS Königsberg was based at Dar-es-Salaam in German East Africa, now Tanzania. She was armed with 10 105mm(4.1 inch) guns and was designed for 24 knots, making her significantly faster than the three old cruisers on the British Cape Station; HMS Astraea (20 knots, two 6 inch and eight 4.7 inch guns), Hyacinth (19 knots, 11 6 inch guns) and Pegasus (21 knots, eight 4 inch guns).
On 31 July Fregattenkapitän Max Loof took Königsberg to sea in compliance with his orders to attack enemy shipping at the entrance to the Red Sea. Pegasus saw her leaving port, but could not keep up with her. Neither could Hyacinth, which encountered her in the dark two hours later.
HMS Astraea bombarded Dar-es-Salaam on 8 August in order to destroy its wireless station. The Germans, fearing invasion, scuttled a floating dock across the harbour entrance, trapping the liner Tabora and the collier König inside, and preventing Königsberg from entering.
Königsberg was bedevilled throughout her career by difficulties in obtaining coal. The Hague Convention entitled warships to refuel at neutral ports. A ship could visit each port only once every three months, but could take on enough fuel to return to the nearest port in her home country. This meant that a German ship could entirely replenish her coal supplies on each visit.
However, the British bought all the coal supplies in Portuguese East Africa, the only neutral source available to Königsberg. This left her having to coal from small German colliers or from captured ships. However, she managed to take only one merchant ship, the liner City of Winchester, which she captured on 6 August and sank a week later.
The British Official History says that she ‘must have had a narrow escape from the Dartmouth‘, a modern light cruiser armed with eight 6 inch guns and capable of 25 knots, around the time that she captured the City of Winchester. She then overhauled her engines in the secluded Rufiji Delta, and the British heard nothing about her until 20 September.
Pegasus was then at Zanzibar, repairing problems with her machinery. At 5:25 am the armed tug Helmuth, a captured German vessel that was guarding the entrance to the harbour, challenged a ship that was heading for an entrance forbidden to merchant ships. The ship, which was Königsberg, raised the German ensign and increased speed. Helmuth failed to warn Pegasus.
Königsberg opened fire at 9,000 yards, immediately straddling Pegasus. The British ship fired back, but her shots fell short. After eight minutes all the guns of her broadside facing Königsberg were out of action. The German ship ceased fire for about five minutes, but then began firing again, before leaving half an hour after opening fire. She sank Helmuth on her way out
Pegasus was then still afloat, but capsized after an unsuccessful attempt to beach her. Naval-History.net lists 34 men killed and 58 wounded, four of whom later died. Königsberg also destroyed what turned out to be a dummy wireless station. However, she made no attempt to sink or capture the collier Banffshire, which carried several thousand tons of coal, or to damage the lighthouse or cable.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. i, p. 155.