Answer:
Soldiers fight it but never changes.
You fill it and it empties, A metaphor for plenty, Depending on what half you see, It's either full or empty
What is seen in the water and in the sky? It is part of the rainbow and can be in your eye
What has a neck and no head, two arms but no hands?
No sooner spoken than broken. What is it?
I am the fountain from which no one can drink. For many I am considered a necessary link. Like gold to all I am sought for, but my continued death brings wealth for all to want more.