An army runs on it's stomach, tanks 'n trucks 'n planes run on gas 'n spares and guns and bombs kill - aka POL and LOG.
Each nation for each type of division has a daily POL and LOG needs requirement tabled according to it's posture - a daily shopping list varied according to the social activities and guests expected that day, in a manner of speaking. Also true is that some nations have armies which can exist on less than others - more anon.
This POL and LOG is generated by production centers, accumulated into a pool (on paper) and then pushed out to the units in the field along Supply Routes which have to be reliable transport assets such as railroad systems and good road networks - air supply is too little too late at exorbitant cost and risk. The Berlin Airlift is a well known exception but it was only a short distance and the Reds weren't shooting.
All armies do it, and all armies mess it up.
Of course it doesn't help if the Route passes through unfriendly neighborhoods as this magnifies the mess e.g.:
- sabotage - especially prevalent and successful in the Western Front such as wrong routes and bills of lading resulting in U boat propellers going to Panzer Divisions in Russia and their ammo going to U boats! Also Panzerschrecks in Normandy and Fausts in Russia where the ranges compromised effectiveness.
- partisans - famous on the Eastern Front and to a lesser extent in the West requiring units to protect the Routes which in turn uses more supply and can't be everywhere at once
- interdiction - mostly air attack which requires air superiority to succeed tactically against the Supply Routes or strategically against the production centers and transport nodes a la Bomber Harris type stuff but please pick the right types of targets.