The Complete Newbie Guide

Game Versions

Hearts of Iron 2 has had many versions . The original game is referred to as HoI2, or "vanilla" and was released in 2005. This game had a series of patches that ended with version 1.3. An expansion pack called Doomsday or "DD" was released in 2006, and also had a series of patches that ended in version 1.3. Doomsday is a standalone and thus does not require the original HoI2 to be installed. Finally, Armageddon or "ARMA", is a booster pack for Doomsday that was released in 2007, and has had a series of patches that have ended in version 1.2. Since early 2009, there is a 1.3(beta) patch for Armageddon which fixes all known bugs and introduces a new espionage system. It is being developed by the community but with the blessing of Paradox.

Armageddon is the latest game version and includes all features of the previous two. In the Anthology pack, Armageddon is a standalone (and thus does not require any other game version to be installed) while as a separate game it is an add-on for Doomsday and thus requires Doomsday to be installed.

Choose Game Type

In order to start on the right foot select a game type that makes the most sense for your learning style.

Tutorials: These are a good starting point for the real basics. Remember that those tutorials have not been adjusted to the expansions and thus may result in errors when playing Doomsday or Armageddon.
Battle Scenarios: These are good for limiting complexity, and learning the basics. The Spanish Civil War scenario which is a fun, eventful little package that will introduce you to all the fundamentals.
1936 campaign:
- For a peaceful, simple game of just the basics, Brazil is a good choice.
- For a start at war, USSR is an excellent choice because it is possible to defeat Germany without much skill.
- As a general rule, trying to reverse history (German or Japanese victory) is more challenging gameplay than a victory with the Allies.

Walk through

Some step by step guides have been created to help new players:

Germany, 1936: "How the heck do you play this game?!" A simple tutorial AAR

General Overview

Intelligence

Available in HoI2, Doomsday:

Using the Intelligence system is not essential for gameplay. Beginners should ignore it for simplicity: some advanced players ignore it due to its relatively low impact/benefit.
Before performing any mission, it is a good idea to set up 10 spies in the target country (which also includes your own), as spy mission success chance depends largely on the number of spies and failed missions make you lose one spy. In Armageddon 1.3beta the espionage system has been changed drastically: there failed missions don't have any negative effect apart from the money lost, so you can happily spy away.
It helps to remove all AI spies in your nation, as they can disrupt production, etc. (Not possible in Armageddon 1.3beta.)
The most widely used feature of the spying system is to steal blueprints.

Technology

Always use as many research slots as possible.
Avoid researching technologies whose historical year is in the future due to penalties to research time.
Match the specialties of the selected tech team with whatever you intend to research.
See the Research guide for more information on how to maximize research.

Production

Serial runs provide bonuses to time to complete due to the Gearing_Bonus . Thus, a serial run of 10 infantry is much quicker to produce, and is much easier to manage as a player. Since cancelling an ongoing serial yields no negative consequences, it is even feasible to order each build as a 99 serial and simply abort it once you've produced enough. That way you'll get your builds done the quickest.

Diplomacy

Take a look at the Domestic Policy FAQ for advice on changing your domestic policy sliders. There are definite directions you want to move your nation to accomplish certain goals.
Examine your resource situation carefully, and if you are tight in anything, scour the world for trades to build up stockpiles in it. Italy and Japan have a particular need for this monitoring. Once war starts your national trade with nations who are non-neighbours will drop sharply in efficiency, so build up stockpiles while the world is at peace.

Military Basics

Your leaders have a rank which determines how many divisions they can command. To increase the rank, simply press the "promote" button in the leader selection window. This will cost the leader in question one skill point and it will reset his experience to zero. Keep in mind not to over-stack leaders: which means a Major General should only have 1 division, a Lt. General should only have 3 divisions or less, a General should have no more than 9 divisions and a Field Marshal should be content with commanding no more than 12 divisions.
Enabling the Auto-promotions option for your leaders will give you "free" promotions, without those leaders losing skill level.
Always be mindful of the terrain and weather you are fighting in (hills, plains, swamp forest, mountains, rivers -- mud, winter, rain, etc): it makes a huge difference.
Units like Marines and Mountaineers exist for a reason: they are masters of certain terrain types and/or mission types such as amphibious assaults.
Garrisons with MP brigades are useful in order to reduce partisan activities (which lowers your Transport Capacity, or "TC"). Units that are in good supply fight better.
Always remember to check your time of day and weather before an attack. Badly timed attacks can wear down even the best prepared attacker.
Time your offensives on a strategic (long-term) time-scale. For example, attacking the Soviet Union as Germany in winter will wear you down for little gain, and almost eliminate the chance for the all-important encirclements. Winter warfare is no fun, just ask the German general staff.
Encirclement is the only practical way of 'killing' enemy units. Plan strategic traps such as cutting units off against the coast and try deep striking tank attacks to get the enemy units behind your lines.
If your attack (or defense) is going badly, or even if it's stalled call it off (or retreat) and think of a new approach. Your units will keep their valuable organisation and strength, and will be ready for the next attack (or counter-attack).
Use HQ units for any large attacks to prevent over-stacking penalties, and use support attack from other provinces to bring an overwhelming force against the enemy.
Remember to give your HQ units general or field-marshal leaders, or else the HQ units are effectively useless.
Don't be afraid of trading land for time or strategic advantage. On defensive fronts consider letting the enemy take a front-line province if your line is attacked by a superior force, and then counter-attack from the flanking provinces using the unit(s) you cunningly withdrew. Then the enemy gets a 20% penalty and things will probably go badly for him.
Check the detailed combat status of your (and enemy) units by reading the tool-tip on the unit in the detailed battle view. This is a neat summary of the factors affecting the combat and a great way of learning when and why you have the advantage or disadvantage in battle.

Things to ignore

Don't worry about convoys. Convoys are important for amphibious landings and maintaining overseas supply, but are safe to ignore for a new player.
Don't worry about leader assignment. Switch on auto-assign and the AI will find suitable leaders to leaderless stacks at midnight every day.
Air Force and Navy are best ignored for beginning players.