I'll go over grammar first, with what I found and can explain. Some of these were already pointed out by lx_theo, which I apologize for repeating.
It has been thirty long years since the fight had become, and many were being to question if it was truly worth it.
"begun" and "beginning"
Its because we came together all those years ago with a single goal in mind.
"Its" is the possessive form. "It's" is the contraction for "it is".
Many cowards found bravery, the weak found strength, and children had become grow.
"begun to"
The dieing fire was reborn.
"dying"
Those original seventeen of the first fight through thick and thin stood by to take the first retaliation in dedication to the cause.
This sentence was rather difficult to read. It's the "through thick and thin" that's throwing me off. Rewriting the sentence to say "Those original seventeen of the first fight stood through thick and thin to take [...]" It sounds better to me that way.
You were; a Spartan.
Random semi-colon. Take it out.
I'd also put this sentence on it's own paragraph to add more power to it, but that's only because I don't know how this continues. So I'm treating it like the ending line.
As for writing style, it's hard to say because we're not really sure where this takes place in the story, or what significance it has. Is this right from the start? Or do we have more information before it or after it? Like is this just basic back story before the main plot, or rather important? (If it's the latter, I'm going to say show more than tell. Develop it more.)
In general, I'd suggest maybe finding a beta reader to help you with the grammar quirks and the few oddly-worded sentences. This'll help clean up your story and make it a mite easier to read and understand.
Other than the grammar, there's not much else to comment on. Hope this still helps, and is what you were looking for in terms of "helpful criticism".