I recently came back in the game and have been playing Axis-aligned Netherlands. I don't remember supply being this bad from playing before. Basically every conflict has had both sides bogged down with not enough supply to push. It's been bad in China, as you would expect, but also plenty of times in Europe and definitely the United States. Late-game America has too many divisions and it's impossible to match their numbers and stay under supply. Most importantly, the AI doesn't seem able to handle it at all. Germany put 150+ divisions in a port that can only handle about eight? I'm used to using 24 division armies with my generals, but there is nowhere I can use them without them suffering from lack of supply. My construction has been all ports and infrastructure but it never seems to be enough. I'm pretty sure if I split the armies more, it won't matter because my "allies" will just flood the area with divisions. They've been fighting for the same province in New Brunswick for literal years because of bad supply. Most of the game I have just had my armies leave because I was just a detriment to the offensive. It's not a fun experience.
There is no defense of AI behavior.
The (start of the) issue is that AI never evaluates supply situation out of the moves it makes combined --- it evaluates supply on an update basis but leaves the troops that it has ordered to move as is.
To get out of your particular situation, can you withdraw from the faction or something? I can't remember how vanilla goes these days.
Have you tried opening up multiple fronts and enlarge your beach head quickly before AI troops can move across the oceans? At least that makes sure you are the owner of the beachhead and can therefore build max level naval base for each front. After that, you can tangentially improve supply by air drop from nearby islands. (Newfoundland for where you are. Carribean islands for Florida/Mexico. Vancouver Island iirc for west coast.)
You can also disable supply to Nova Scotia by marking the naval zone inaccessible. Starve out your allies and let the enemy kill them. Without divsions, your allies won't be in the way.
Lastly, you need mods. Various mods allow you to coordinate allied troops a bit more. I would not play HOI4 without at least
Expert AI. (I still wouldn't play it without BICE or something similar. Without Expert AI though, vanilla HOI4 simply doesn't work after each country's initial playthroughs. And half of those playthroughs are trivialized due to some game design/balance issues.)
