That's actually a whole lot better than I had expected it to be, though that's not saying much.
But I enjoyed it and that IS saying much.
There is a bit too much ripping off from halo though... kids getting trained from a young age to become space-armored supersoldiers, fighting aliens that are stronger than them, conquering other species to expand their empire? If that does not ring a few bells...
Also, this: "The groaning noise made was spine tingling" is not a good sentence. apart from that, try to avoid using the same word repeatedly, in: "That noise will be imprinted in your minds,", I would replace 'noise' with 'sound', just to make reading it a bit smoother.
Since you are talking about aliens (right?? I assume so since they are about to invade Earth, using a sci-fi portal thing) don't refer to them as 'men', try soldiers or warriors or whatever the hell their race is called. additionally, to avoid confusion, refer to the ground on planets other than Earth as soil, dirt, mud or just ground.
enough nitpickery, on to the bigger stuff.
Aliens conquering other planets because they have a 'need for a larger empire' is not a good story element. It makes it far too easy to fall into the generic space marine crap that we've seen way too much of already, try to give them a decent reason for going to war, like they're all going to die if they don't, they have an incredible food shortage or someone is going to kill their puppies. Desperation ads a layer of atmosphere to your story and it gives the main characters a goal.
Right now it's looking a bit too much like the 'they're evil because someone needs to be' thing. and that's bad.
If you're gonna take this stuff further, make sure you make the characters well, this piece is good for drawing us in, but if you stay at this level for the rest of the story, I'd chuck it in the bin after two chapters.
But despite all the nasty things I just said, I like it, and would read more. If you're going to continue on it, let us know!