It is labeled a "grand strategy game" for good reason. The manpower issue is a big concern for me, particularly once you conquer territory.
To quote an earlier post (I'm lazy and don't want to retype):
It doesn't allow for recruiting in occupied territories, something the Germans took to extremes.
Using my "England" picture in post 535 I'll try to explain the main screen and give you an idea of what you're controlling. WARNING: Long post follows
Top bar:
National Unity (trivial unless you're losing)
Political Power (used for selecting ministers (15 total) plus recruiting, manufacturing, and exports)
Population available for recruiting
Manufacturing
Nukes (total number and completion percentage to the next)
The Green, Blue, and Red stars are points used to create and modify division, ships, and aircraft. The last is available convoys. The stars are earned through combat or ministerial modifiers. Convoys are manufactured and something to watch, especially as your empire grows.
Second row in the top left, the colors are from a mod because the default color is light grey.
Blue/ Research. Controlled by slots and modified by researched technologies.
Red/ Diplomacy. Use this to influence other nations
Turquoise/ Trade. In conjunction with the modifier above this allows you to trade for scarce resources (oil, steel, chromium, tungsten, aluminum, and rubber) used to manufacture equipment. For instance, aircraft require various units of oil, aluminum, and rubber.
Brown/ Construction. Infrastructure, radar, etc.
Green/ Equipment Production. Tanks, planes, ships, artillery, etc.
Purple/ Unit Production and Customization. Units have up to 5 or 6 Support slots (Engineers, Hospital, Logistics, Recon, etc.) and something like 16 units slots for men, tanks, artillery, anti-tanks, etc.
Yellow/ Equipment/ Spares On Hand
All of the above are controlled by the player through their respective subscreens. For example, Research: Click the button, click a research slot, scroll through8 or 10 tabs of equipment types and doctrine, select one to research, and move on.
To the right you can set up theaters (I need to look into this because I run them all at once). Map locations provide you with a status. In this one, Greenland says I have a supply shortage and there's a naval battle in the Java Sea.
Bottom: my various armies. Those are customizable in content and the icon. The generals come with hardwired traits like Panzer Leader or Engineer (a river crossing bonus) and can earn additional attributes through combat. If you run out of those generals you can burn political power to select a new general, but one that has zero attributes at the beginning.
If you zoom in you can see the tiles representing available units and what you're fighting. I'm going up against 2 UK leg infantry, 1 Greek mountain unit, and 1 UK para unit. The Green and Yellow bars represent unit integrity and strength. If you select a unit you can hover over those for feedback. On the tiles you'll see a shield-like icon. One is an outline and another is filled. This represents a unit's entrenchment. That obviously gives a unit defensive buffs. The color on my 6 units of mountain troops is one used to identify the army which you can in the icons at the bottom of the screen.
Build and customize your units, select a point of attack, and "git r done." 80% of the work is getting to the point of attack and the last 20% is determining where and how.
Thant's just your overview of your GUI, not an in depth concerning the mechanics and capabilities.
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. I fall for it everytime. 
