What an unfortunate situation. We all make mistakes sometimes, I hope his weren't fatal.
Traction is key here. I learned about metal on metal the hard way forklifting a milling machine up an incline many years ago in a way I'm not going to forget (luckily it was just the mill that needed repair)! never expect traction from metal on metal, even some sand might have saved this but really a heavy rubber strip (I'm thinking scrap conveyor belt) would have been ideal. Nylon webbing (think heavy cargo strap) in a pinch or throw down some plywood, just anything to get some bite. If you can't find traction then expect it to slide and plan accordingly, load binders or a winch gradually released would have worked. If you are transporting armor, you can save yourself a lot of heartache putting derailers on your trailer so it can't slide to the side, just screw off some 6x6 inside the tracks or weld some rails along the outside.
Still, it's another sobering reminder that big vehicles don't care who they hurt so be careful out there.
