‘Their’s but to do and die’
Celebrates the British soldier who will follow his order, even if it ends in certain death
The conjunction ‘and’ makes very clear that death is inevitable, but duty and patriotism to the soldiers are more important
He could have used the conjunction ‘or’ but he avoids this to show that when you do you will die, not factually true as over half of the soldiers were killed. Much more survived than Tennyson thought