Germany is about to kick things off with Poland. What should be my plan as Italy?

>>1991997
If you're playing as Germany and moving in, you'll have to build supply hubs and railways. Infrastructure will help, too. You'll probably be fine on trains but make sure to check.
If that's going to take too long for you, then see if you have any transport planes in stock. You can give them a mission to airdrop supplies to a region.