Consider how much food, toilet paper, water, laundry detergent, etc. you use in the course of a week. Multiply that by the number of people in your group. Now multiply that by the number of weeks you want to go play.
That gives you an idea of your supply needs for an operation. Not having it squared away ahead of time makes you vulnerable, because if you have to rely on the charity of others bringing it through hostile terrain. They might not have it to give, and it might get intercepted and never reach you.
Back in the day, I had the privilege of being friends with the late Roger Cravens of Georgia and Missouri. Those of you who were involved in this stuff back in the day should remember his postings to PIML and the various Fidonet groups. Roger also happened to be a major influence on the SIGINT/COMINT portions of the book and workshop.
Roger was a Vietnam Veteran, who in the later part of his service was involved with S3 on the division and corps level. For those not in the military, S3 designates the staff function for operations planning.
Many years ago, Roger turned me on to a two (now three) volume document that changed the way I looked at things. In the interest of preventing further screw-ups, I will share them with you.
These manuals constitute the entire planning process for the US Military. They are referred to as JOPES - Joint Operations Planning and Execution System. They are unclassified, and are the real deal. As was the guy who turned me on to them.
The biggest volume is over 600 pages long. They are a long, dry read. They contain invaluable information that will help you avoid a half-cocked clusterfuck.