Theirs not to reason why, theirs but to do and die: Meaning and origin

These words are from the poem “The Charge of the Light Brigade” by Alfred Lord Tennyson, after the Battle of Balaclava during the Crimean War.
One of the most famous battles in the history of the British army was the charge of the Light Brigade at Balaclava. This took place during the Crimean War in which Britain, France and Turkey were fighting against the Russians.
Balaclava was a small harbour town occupied by the Allied army, 8 miles to the southwest of the harbour city of Sevastopol. Balaclava was defended by lines of earthworks, built on the hills around the harbour.
On October 25, the Russians tried to break these lines. After defeating the Turks on the heights, they seized their guns and moved down into the plain. Here they attacked the British forces and were driven back across a low ridge of hills crossing the plain by the Heavy Brigade of cavalry.
Then, owing to a mistake in the giving of an order, the Light Brigade initiated a suicidal charge a mile and a half along a valley to capture some Russian guns. Tennyson referred to this valley as the “Valley of death” in his poem. More than 600 cavalry were involved of which more than half were killed, wounded or captured.

“Forward, the Light Brigade!”
Was there a man dismay’d?
Not tho’ the soldier knew
Some one had blunder’d:
Theirs was not to make reply,
Theirs not to reason why,
Theirs but to do and die,
Into the Valley of Death
Rode the six hundred.

So, these words were written to do justice to the bravery of soldiers under orders, irrespective as to whether the orders were foolish or not.
Lots more interesting stuff on the details of the battle and analysis of the orders given, at Wikipedia.
Image from Wikipedia