I read this book between 5 and 10 years ago. I really can't remember any of the story (I've had it on my mind for two days now), all I remember is this scene.
A female protagonist is beside a bridge on an alien planet river with boyfriend/husband when the antagonists try to kill her. The first she knows is loud clangs, it turns out that Planetary Defence has either 'transporter beamed' or 'rapid tractor beamed' a cloaked Armoured Personnel Carrier between her and the bullets.
A very fast firefight happens and the bad guys are gone. She is then introduced to The Planetary Army, this proves to be one guy who has like enhancements and controls loads of mech fighting machines.