A friend of mine was in the 2004 Iraq War. He told me stories about running out of a certain lubricant for Mk19 Grenade Launchers. Apparently, the launchers would not work without it and there was a theater-wide shortage.
Imagine how many spare parts, lubricants, batteries, types of ammunition, medical supplies, etc.. are junking up the supply chain. There must be thousands and thousands of SKUs....
The complexity must be mind numbing.
Now imagine how things are going in Ukraine when Ukraine gets "hand me downs" from 30 or so nations while also fielding Soviet era gear and developing its own gear.