You can also speed-up war justifications by taking advisors or focuses with "subversive activities" modifiers. For example Ribbentrop for Germany (-25% shorter justifications), or "NKVD Primacy" focus for USSR (-50%).
My tip as a begginer to begginers: If you play minor nations against AI, you can make you infantry much stronger by adding a medium or light tank with max armor upgrades, it will give you armor, that helps a lot.
If I had to give a tip to beginners it would be to focus on industry before politics and army that way you can grow your army, navy, and airforce to their full potential.
@hammotimee a generally good tip for most minor nations but if you're like me and trying to play france as the little entente then you know you'll be forced into war at a certain point so getting the alliances is more important
Big tip LISTEN to all the tips from Dommy TommyKay, I put in 1000 hours and holy hell (not getting me this time youtube) they can help, don't just copy, also one from me, Supply is not consumed by the division but the battalions that make. so you cannot just make 25 infantry battalions in a division to reduce supply consumption.