Battle of Jassin
The Battle of Jassin (also known as the Battle of Yasin, the Battle of Jasin, the Battle of Jasini or the Battle of Jassini)  

was a World War I battle that took place on 18– 19 January 1915 at Jassin on the German East African side of the border with British East Africa between a German Schutztruppe force and British and Indian troops. Jassin had been occupied by the British in order to secure the border between British East Africa and German territory, but was weakly defended by a garrison of four companies of Indian troops, commanded by Colonel Raghbir Singh  
 and numbering a little over 300 men. Colonel Raghbir Singh was killed during the battle. The German commander, Paul Emil von Lettow-Vorbeck, decided to attack Jassin in order to prevent further danger to Tanga, which lay more than 50 kilometres to the south and had previously been successfully defended against a British attack. Nine companies of Schutztruppe with European officers were gathered for the assault. Immediately after the British force surrendered, British Captains Hanson and Turner were taken to see Lettow-Vorbeck. He congratulated them on their defence of the town before releasing them on the promise they would play no further part in the war. Aftermath
Brigadier-General Michael Tighe arrived too late, just hours after the surrender to support the British at Jassin. Although the British force surrendered, Lettow-Vorbeck realised that the level of German losses of officers and ammunition meant that he could rarely afford confrontation on such a large scale and would need to make use of guerrilla warfare instead—he turned his attention away from seeking decisive battle against the British, concentrating instead on operations against the Uganda Railway. The British response was to withdraw and concentrate their forces in order to reduce their risks and make defence easier. As a result, the invasion of German East Africa was postponed for some time. References

 Detailed report on the Battle of Jassin by Dennis L. Bishop and Holger Dobold

1915 in German East Africa
Battles of the East African Campaign
Battles of World War I involving Germany
Battles of World War I involving the United Kingdom
Battles of World War I involving British India
Conflicts in 1915
January 1915 events