> It moves the production line closer to enemy action to destroy it,
Please read up on how warfare works. There is The Front, The Road to the Front, The Border, and usually miles away from all that: Where vehicles refuel.
Why on would anybody have a 3D printer on front, instead of just having one in a supply area?
Also: I don't think you understand just how much Logistics a 3D printer removes, if it can print ANY reasonably replaceable part.
Instead of a of a forward supply depot with random parts that will never get used, there is a smaller depot of materials. And if there is demand: A even smaller cache of pre printed parts, to avoid waiting for printing.
>We can transport just about anything anywhere in the world in 24 hours
Package arrives for mail: Will be shipped in next 12-16 hours. Repeat for every single relay.
So army base -> county mail system -> overseas shipping location -> sea/air travel -> arrival on right continent -> and a few more chains of relay
I expect minimum time for that to be several days, if not a week.
If its a priority thing, thats still hours to get shipping going, shipping overseas quickly take half a day, and more is taken to get it on location. It will take over 24 hours unless you start risking air drops in field.