You will be given a definition of a task first, then some input of the task.
Generate a correct and concise answer for the question based on the words in the context.

Context : The 21st Army Tank Brigade was an armoured brigade formation of the British Army active during World War II. The brigade served with the British First Army and the British Eighth Army during the fighting in Tunisia and Italy.
Question : Who did 21st Army Tank Brigade serve for?
Output:
British Army