Tanga.
The "Battle of the Bees".
Tanga finished Aitken, although it was not ALL his fault. You can't just park an obvious invasion fleet offshore and sit there until the (extremely competent) German C.O. comes ripping down the railroad from the interior.... especially not after the Navy had come ashore and demanded that the enemy surrender, then spent the following days sweeping for non-existent mines. Aitken never commanded again. Wapshare, commanding a good chunk of the Indian Army contingent, was named in rather a saucy song and spent the rest of his life trying to clear his name.
The invading "British" force was overwhelmingly brown Indians. The defending "German" force was about 95% Black. No-one can say they were not good soldiers. After Tanga, once the Indians got their war footing right, they fought well through the appalling "Bush War" which ensued for the next 2 years, working with the King's African Rifles (nearly all OR were Black, with British officers) and troops actually pulled from the European theatre. The "Germans" fought well and turned up to surrender in good order at Abercorn, Rhodesia on November 25, 1918: two full weeks after the war had ended in Europe.
Tanga cost the defending Kaiserlich Schutztruppe 16 Europeans and 55 Africans killed in the action, a total of 76 wounded, for a grand total of 147 casualties.
It cost the British a confirmed 360 KIA and 487 WIA, with German estimates running as high as 2,000 altogether.
In the end the tiny German force was defeated by more than 180,000 Allied troops at an expense to the British alone of 72,000,000 pounds Sterling, roughly $22 BILLION in today's money. This was precisely what the German commander, General von Lettow-Vorbeck wanted to happen; all he wanted to do was draw men and materiel away from the European Theatre.
There are a surprising number of good books about the East African campaign, including the memoirs of the German commander and a book by Meinertzhagen showing the British side of the story. Perhaps the most engaging is "On to Kilimanjaro" (1963) by Brian Gardner. Your local second-hand bookstore can get you a copy.