Earth invaded

The Alien Invasion was unexpected and devastating, with their protective force-shields, the K'Thraa stormtroopers seem invincible and it looks as if earth is doomed. But by a miracle of chance First Sergeant Hood finds a flaw in the K'Thraa armour, a way in which he and his tiny army may be able to fight back.
But is it really possible for such a small band of soldiers with their primitive weapons to defeat the sophisticated aliens? with Hood in charge, there is a chance.