Failed to get "theirs" - does it refer to "the duty of theirs/the soldiers"?

Ah, " The Charge of the Light Brigade".

My heart still palpitates when I read that.

Yes, it refers to the soldiers' duty.

"The Charge of the Light Brigade"

by Alfred, Lord Tennyson

Half a league half a league,
Half a league onward,
All in the valley of Death
Rode the six hundred:
'Forward, the Light Brigade!
Charge for the guns' he said:
Into the valley of Death
Rode the six hundred.

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

Cannon to right of them,
Cannon to left of them,
Cannon in front of them
Volley'd & thunder'd;
Storm'd at with shot and shell,
Boldly they rode and well,
Into the jaws of Death,
Into the mouth of Hell
Rode the six hundred.

Flash'd all their sabres bare,
Flash'd as they turn'd in air
Sabring the gunners there,
Charging an army while
All the world wonder'd:
Plunged in the battery-smoke
Right thro' the line they broke;
Cossack & Russian
Reel'd from the sabre-stroke,
Shatter'd & sunder'd.
Then they rode back, but not
Not the six hundred.

Cannon to right of them,
Cannon to left of them,
Cannon behind them
Volley'd and thunder'd;
Storm'd at with shot and shell,
While horse & hero fell,
They that had fought so well
Came thro' the jaws of Death,
Back from the mouth of Hell,
All that was left of them,
Left of six hundred.

When can their glory fade?
O the wild charge they made!
All the world wonder'd.
Honour the charge they made!
Honour the Light Brigade,
Noble six hundred!

Thanks.
Half a league = the six hundred?

No.

A league is a unit of measure for length. Half a league is about one-and-a-half miles.

The six hundred is in reference to the number of cavalrymen.

Sad. Cannot find it out in Oxford:
http://www.oxforddictionaries.com/definition/english/league?q=league+

Look at "league²"

Does "
Half a league onward,
All in the valley of Death
Rode the six hundred" mean:

(1) The passage (with a distance of half a league) is "the valley of Death"?
(2) The six hundred men rode onward/in the valley?

1)Yes.

2)The six hundred men rode through the valley, charging for the guns.

Thanks.
Does "charging for the guns" refer to "charging directly facing the muzzles of the guns"?

The specific line is:



Ultimately they are not ordered to charge "the guns," per se, but the bastards who hold them. A later line (after reaching their destination) is:



Literally they are charging an "army" with men possessing guns. The order is to head straight for them and slaughter them.

In this context, "for" has a dual meaning:

1. in that direction and
2. for the purpose of eliminating the "guns"