'The Cavalry' has arrived!................and then it got destroyed?

Jun 2012
Often in movies, we have a brave company of soldiers fighting far beyond the reach of any quick reinforcements. Forced by desperation, they inflicted a surprisingly high number of casualties upon the main army of the enemy. Afterward, those brave souls either all died or the reinforcements ended up miraculously arriving on time and saving the survivors. I'm sure that there are plenty of movies that subverted this trope, but the thread is not about films.

In real history, were there any notable cases where the reinforcements under the central commanders themselves arrived on time to help the brave heroes, but both were still pushed back/defeated/crushed by the enemy regardless?
Feb 2011
Battle of Stamford Bridge 1066- Norwegian reinforcements - proceeded at the double from the ships they were guarding and arrived exhausted at the battlefield, to be destroyed.
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Jul 2018
Confederate forces progressively grew on the battlefield during the day but, at the end , the Union remained in control of the battlefield. They didn't manage to do anything with the victory, but I believe this may be one of the cases. I speak about it briefly in the video below.