So I get that you can only claim power for your house card during a military challenge, my question is that the wording of the card says

"After you win a military challenge, claim 2 power for you house"

Does this then include military challenges where I am the DEFENDER and win?

Yes.

If you were only intended to claim power as the attacker, the card would have said so.

Thanks, I thought that was the case but one of my playgroup was convinced that you could only gain power as an attacker.

That's one of the nice things about this agenda - that it works as a deterrent for making MIL challenges against you unless your opponent believes they can win.