I suppose it really depends on what type of unit you're in and where you're going to be deployed.
I've been in quite a few different units (Armor, Medical, Infantry, Combat Engineer, and Artillery), and in some all I needed was a Swiss Army knife, while in others I used every tool in my hummer's pioneer kit frequently (Hummers have a pioneer kit that includes a full size ax, full size pick, and a D-handle shovel).
Overall, I think a small ax will be your best bet (like a Gerber Sport Ax). It's can split skulls as well as trees!
Combine a small ax and a multi-tool and there should'nt be much you can't handle.
Semper Fi and Drive On,
Allen.