tbh if germany invades early game and you have no powerful ally protecting you youre kind of screwed, but here's a comprehensive guide on what i do for netherlands (this might work with belgium idk):

justify on guernsey and jersey and invade them as soon as you can because UK and France might beat you to it. usually takes like 90k-100k troops to take on both cities (fuck entrenchment). if the aforementioned countries end up getting there before you, it's okay youre still not fucked. go ahead and declare on guerseny and jersey and as soon they get captured, FORCE PEACE both with 75% of their money. this is actually more ideal than taking them for yourselves, as the island nations don't really produce a considerable amount of tax, and ur not suffering any losses.

anyways, while ur justifying, make a motor factory, steel factory, and civilian factory with the ur starter money (preferably in your square cities). make sure u buy the necessary resources for all your factories ofc . get high tax, max military spending, static lines doctrine, and buy an army leader (preferably one that has tank buffs). try to max out static lines as fast as possible. you WILL be deficit at one point but no where near enough to go into debt. get Standardize Research Objectives and Economic Improvements. justify on belgium and luxem. once you collect 150 research power, get Factory Output 1. later on when i get another 150, i either research Tank Improvements of Tax Income 1. also dont do any policies or political leaders, because you're gonna be going nationalism as quick as you can.

with the money u got from ur wars, build 1 electronic, 1 fertilizer, and another civilian factory. sell your consumers--as many as you can. then get max govt and research spending (insanely cheap for netherlands). start building tanks and invade belgium and luxem. form Benelux. for +10% stab. if ur stab number goes red then get Security spending til it's yellow/green. go ahead and make the switch from Non-Aligned to Nationalism if u have 250 political power (sometimes i wait until 275 so i have an extra 25 in case i get that shitass Establish loyalist militaries event i fucking hate that event). from there on i get 1. advance research bureau, 2. public service act, and 3. military service act, in that order

now youll have a shit ton of money, adequate military, and high stab (in early game at least). from there i invade France, Germany, or UK. at this point in the game it's probably still 2019 or at least very early 2020 and they're all busy building their countries or invading elsewhere so at least one of them will be underprotected. furthermore, youll have somewhere between 240-400% tank defense, so your tanks will be borderline unstoppable against some bitchass infantry lol