When invading the US, supply is critical and (initially) the only way of getting it is by convoy across the atlantic. The size of the port you take, limits how much supply comes in and thus how big your army can be. So one trick is to pick one main port, but invade multiple ports along the east and south coasts. You just need a few Marine divisions (I typically use 2 Landing craft and 3 Marine Divisions) and work your way through the Ports one at a time. After each secondary port is captured, land a single Infantry corp of 5 Infantry divisions and corp HQ (or create the HQ after landing). Withdraw your Marines for the next Port invasion. Your infantry divisions push out and take a couple of defendable hexes, ideally along river banks and in woods/hills and then dig in.
Why?
1. The AI pulls out 10 or 15 divisions from where they are really needed to travel over and hem in each of your static Infantry corps. Eventually that can be as many as 60-90 divisions, usually Mechanised divisions, just babysitting 25-30 of your Infantry divisions. Rarely will they attack, just dig in alongside yours.
2. You force the US navy to rebase further and further away from your main invasion. Ultimately forcing them into the pacific where they can no longer intercept your main convoys.
3. Each port captured becomes the supply source for the nearby garrision. Infantry divisions need little supply and no fuel. There are few if any supply issues with them. Often you don't need convoys continously supplying them if the ports are big enough, one convoy every week or two can be enough. If you can handle the micro management of them. Often you capture a load of supply on the port hex too, which can last them some weeks without any convoys.
Your main invasion force can be smaller as it now faces a much weakened enemy. Your first goal should be to capture the US supply source (Washington I think) and a massive stockpile of supplies and fuel, then you are golden to ship in the rest of your army. Make sure you have the creation of resource convoys set to manual before you take this supply centre. If you set up a resource convoy to return resources back to Europe make sure you untick the Supply and Fuel boxes otherwise the first convoy will remove those stockpiles.
Be wary of linking up with Port beachheads, because at the same time you link 2 supply sources. When this happens, the game dismantles both supply sources and picks a new port province to become the new supply source, not necessarily either of the previous supply sources. I think it tries to place the new source based on the optimal (smallest) sum of supply chain lengths, but the results are often unsatisfactory. A period of supply disruption follows as the old stockpiles are moved to the new supply source.
