[Polydamas]

Originally Posted by Michael Thayne

Hmmm. Maybe require TS equal to say 5% of the LS then? Possibly divided by length of the supply in hexes or something like that?

Michael, I would say it depends on the terrain and the infrastructure. Blocking a one-lane wagon road over high mountains is easier than blocking the open steppe. Blocking supplies to a big army is easier than blocking supplies to a small one (the big army may depend on a single railway).

In many cases there will be troops protecting supply lines which are beyond the scope of GURPS Mass Combat, which focuses on the forces commanded by PCs. Its only with the telegram and the trained general staff, and then really with the portable radio, that commanders can execute control over troops more than a few hours' ride or sail away. Usually, the PCs force will be part of a larger operation and transport network. And do you really want to worry about the location of storage depots, the timing of convoys and columns of replacements for casualties, and all that technical stuff which determines where these troops are at a given moment?

Originally Posted by Michael Thayne

Note that the question here is not just about how to handle PCs cutting supply lines—its also about how to handle PCs declaring they wanna pull a Sherman's March. If supply lines are not actually cuttable, and what matters is just having the money to pay for a logistic force, the maybe the difference between actual supply lines vs. extensive preparations to live off the land is just a matter of roleplaying with no game-mechanical effect. If supply lines are cuttable you have to come up with a better answer.

A pretty common premodern logistics strategy is telling a bunch of merchants what you will need and letting them sort out the details. That has two big limits: there still has to be enough food and fodder within transport distance of your army, and they can't handle reinforcements or specialized things like ammunition so well. In those cases, I would treat wherever those merchants are buying and stockpiling as "home base."

A pretty common premodern logistics strategy is getting supplies, animals, valuables, and personnel from locals (occasionally even paying them for them!) A big problem is that you can't get blood from a stone: there is only so much available, and only so much can be gathered over typical low-tech rural transport networks.

If I were writing foraging rules, I would probably let a reasonable roll support TL 0-4 forces up to one order of magnitude greater than the typical local settlement. In Iron Age Europe a few hundred men can roam freely between hamlets of a few dozen people, in a river valley with a tel every few miles or a coastline with a port on every bay you might support up to a few tens of thousands. There would be a steeply increasing penalty for each subsequent month within say a 20-mile radius. The numbers would start to look dicey at TL 5 and TL 6, and impossible at TL 7+, especially for troops more expensive than Riflemen.

You could get a substantial bonus if you are willing to pay cash money and its a society with commerce.

There would be a penalty if its the kind of place where everyone has a walled village, a hidey-hole in the jungle, or a place to bury their grain and the locals have warning and chose to resist.

I would look closely at Sherman's March to the Sea, how many casualties the Union took, what they had to leave behind, and how close they were to disaster when they finally took Savannah. Wikipedia gives the dates as 15 November to 21 December. Mass Combat p. 7 says "These values are measured in $, but in practice – especially at low TLs – many commanders “pay” part or all of these expenses by conscripting civilians, appropriating mounts and vehicles, and foraging, pillaging, and looting."

In a GURPS game, I would have scenes gathering up money, transport, ammunition, negotiating with major NPCs and the President, etc. Then the army leaves Atlanta and its Land Logistics no longer have contact with friendly territory. No later than 1 December, Maintain comes due, possibly adjusted for Maintaining a Force in Bad Terrain. I would roll for how much the army foraged up, but the army would probably drop to Low Readiness or No Maintenance. Fortunately, the Confederate forces have their own troubles, and with some good Strategy rolls, clever problem solving by the character with engineering skills, and interaction with major NPCs the players take the port and the supplies stored there. By January or February the Union has delivered enough Naval Logistics to support the force at Savannah. So the very simple two and a half pages of rules in GURPS Mass Combat can handle this.