Considering Gordoncz's website seem down, and i don't know if it will be permanent, i have decided to upload the recommended mods and tools so people can enjoy the game to its fullest. As well as helping new players out properly, something i failed with my last thread one year ago.

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1. - Getting game to work.
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Older gog.com and retail versions of Blitzkrieg had crashing issues when the game.exe was assigned more than a single CPU core. I'm not sure if this is still the case with the new multiplayer .exe file, but here is how you assign the game only a single processor core in case you experience crashes.

On Windows 7, 8 and 10, simply right click your gog shortcut of Blitzkrieg and click "Properties".
In the 'Target:' field, enter:
C:\Windows\System32\cmd.exe /C start "" /affinity 1 "C:\GoG\Blitzkrieg\game.exe"

Edit the "Start in:" field to point to the game directory ( C:\GoG\Blitzkrieg ) and not the galaxy client ( C:\Program Files (x86)\GOG Galaxy ).

This will tell windows to only assign 1 core to your game (core 0 to be exact.) and make it run error free, proceed to do the same for Burning Horizon - Rolling Thunder:
C:\Windows\System32\cmd.exe /C start "" /affinity 1 "C:\GoG\Blitzkrieg BHRT\Run\game.exe"

Edit the 'Start in:' field for Burning Horizon - Rolling Thunder to make sure it's set to the game directory ( C:\GoG\Blitzkrieg BHRT\Run ).

If you installed the games elsewhere, edit the "Target:" and "Start in:" text so it point to it's correct install location on your system.
Once that's all set, you can enjoy the game vanilla with full compatibility!

Note: If you run the game in Steam, this will not work, and you will have to manually set the affinity for the game.exe for its process in the task manager. (Simply launch game in Steam, Alt+Tab, launch Task-manager, click Processes, right click game.exe, click "Set Affinity" and only have CPU 0 [X]'ed in, click OK, and close task manager and have fun playing the game. This must be done every time you play the game in Steam.
On Windows 10, right click "game.exe" under Apps in Task manager > Go to details > right click "game.exe" > Set affinity > Only leave CPU 0 [X]'ed in and press OK.

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2. - Mod's and Fan Fixes.
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Note: With the release of game version 1.2.2XPLAY (A), the game installation is slightly different from retail version and older gog.com releases. All stuff that used to be under the "Run" folder is now in the base "Blitzkrieg" folder. And the "data" folder is located under the "*\Blitzkrieg\data" folder. Keep this in mind for when you install mods.

Now when you got the game up and running, a essential fix for Blitzkrieg 1 and its expansion is the Stalingrad Ai dll. Blitzkrieg have had many stand alone expansions, some licensed ones by other developers, one of them is 'Stalingrad'. It included a tweaked ailogic.dll that improved the Ai greatly, making sure it cant shoot thru objects such as houses and hit/see your units while you can't hit/see them.

Download link to Stalingrad ailogic.dll:
ModDB link.
Dropbox link to ailogic.zip

Simply place the ailogic.dll in the "*\Blitzkrieg" and "*\Blitzkrieg BHRT\Run" and overwrite the old ones to install it.

Once Ai is tweaked, lets begin talking about mods in general. mods are usually in .pak files, and thus can easily be installed by placing them in the 'data' folder located in "*\Blitzkrieg\data" and "Blitzkrieg BHRT\Run\data". You can easily uninstall mods by deleting its .pak file.

A essential .pak is the Winter Buildings mod, that gives buildings that normally lacked winter versions just that.

Download link to winter-buildings.pak:
ModDB link.
Dropbox link to winter-buildings.pak

For Burning Horizon - Rolling Thunder a recommended mod is the Nippon Japanese Realism Mod by Corpbob with voices prepared by Joseph_Porta. it adds voice-lines for Japanese troops, adds descriptions for Japanese units, as well as adding new squads and equipment for multiplayer.

Download link to Nippon Japanese Realism Mod:
Dropbox link to Nippon_v10.zip

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3. - Mod packs and Fan Campaigns
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There is a lot of Campaign packs made by fans of the original Blitzkrieg, they include a ton of custom units and maps, as well as new buildings and related environmental files. They are also easy to install as they come on a normal installer, but all of these Campaign packs are made for the retail version of Blitzkrieg, so it default suggested install location can be wrong. So simply just ensure its set to install in the right gog.com versions folder and it should be all good.

Pleas note, only have one of these packs installed at a time, as it may otherwise may cause unexpected scripting issues between the mods. So mix at own risk.

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Achtung Panzer!
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My personal favorite, it replaces the original German Campaign with a new custom one, includes a ton of units and skins, and will not disappoint if your a fan of playing as Germany.

Download link to Achtung Panzer:
ModDB link.
Blitzkrieg.be link.

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Motherland Calls
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Is a Russian campaign mod that is basically a Russian counterpart to the Achtung Panzer mod, but also changes the main menu and the battle interface of the game.

Motherland Calls got 2 big addons as well that adds more gameplay. Those are Buzuluk To Prague and Motherland Calls - The Other Side. Other Side allows you to play as Germany during the Russian campaign.

Download link to Motherland Calls, Motherland Calls: The Other Side and Buzuluk To Prague:
ModDB link.

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Other Campaigns
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See their ModDB pages for more information.

Download link to Operation Blau:
ModDB link.

Download link to Plan R4:
ModDB link.

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4. - Optional mods.
===============

Some good optional battle interface mods are GordonCZ Interface and Stalingrad Interface.
The Stalingrad Interface adds the Stalingrad stand alone expansion interface to original Blitzkrieg, while GordonCZ interface is a custom one made by Gordon himself. if your tired of the vanilla one, you can always give these a try.

Download link to Stalingrad Interface:
ModDB link.

Download link to GordonCZ Interface:
ModDB link.

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Credits
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Each mod belong to their respective owners and creators. Each campaign pack installer include a ReadMe with credits naturally, and if i missed any credits for any specific .pak file feel free to tell me and il add it to my post.

I own no right to any of this and simply just re-uploaded it for sake of the community, so no infringements are intended.

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Edit (28 Oct 2018)
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Thank you to urosl for being so kind and linking to alternative download sources for Blitzkrieg content! As well as to GordonCZ for being so kind to re-upload his mods on ModDB!
I apologize for the download links being down, older links from before March 15, 2017 broke due to changes with Dropbox. I have added links to GordonCZ ModDB and blitzkrieg.be pages for respective files and campaigns, and updated my Dropbox links for the other content as backup.
It's also worth mentioning that Blitzkrieg Map Shop is still up in case anyone want to download more maps that isn't available on ModDB or blitzkrieg.be.

Sorry for the inconvenience.

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Tactics and Tips.
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Some general tips to get into Blitzkrieg is to take things your own phase, and to not be afraid to use artillery to soften targets up.

Infantry attacks are very useful in city warfare and to deal with anti tank guns, but they are very fragile, so use the environment to your advantage by utilizing existing trenches and keep using infantry in close combat to minimize losses. Make use of trucks and APC's to move your infantry around quickly, but just remember to keep them out of harms way. Infantry squads with a commander, lone commanders and snipers can use binoculars, use it to see further than normally while staying out of enemy fire. Snipers have very good camouflage, they can stay undetected quite well while sniping, consider dropping one down behind enemy liens when possible to deal with enemy artillery.

Make sure to utilize engineering trucks to their full abilities, they are capable of more than just fixing your damaged tanks up, they can repair bridges, lay mines, dig trenches, place barbwire and other handy things. (Just remember it burns a lot of supplies, so have a supply depot nearby to restock them at.) Right click on engineering trucks to make them automatically repair everything within walking distance.

Make use of abandoned enemy equipment such as anti-tank, AA guns, artillery and heavy machineguns. Capture them by right clicking on them with infantry. They can also be repaired, resupplied and towed by your supply trucks. (Some heavier artillery units may require a heavy half track supply vehicle due to its weight.)

Normal trucks carry ammunition and supplies, make sure to supply your tanks, anti-tank guns and artillery when needed. Just remember to re-stock the trucks form supply depots once they start running low. Right click on them to make them automatically resupply everything within walking distance around them.

You can dig in Tanks, armored cars and artillery as well as anti tank guns very quickly with dirt and sandbags. So if you know you will be attacked, always try to quickly prepare yourself by digging your stuff in to increase their defensive capabilities. (Just remember to make them face towards the advancing enemy.) Right click on your tanks, armored cars and guns to dig them in.

Use bushes to your advantage as they can be useful for keeping anti tank guns and tanks hidden. Small cliff-sides next to roads are great ambush points, as the higher height makes your units hard to spot as long as you don't sit to close to the edge, meaning you can see and shoot them, as they cant see you and return fire until they gotten closer. Bushes and other flora can be destroyed by simply selecting the attack mode and clicking at whatever bush/tree you wish to destroy. (Very handy in town battles, where bushes and other flora may keep your units form spotting any hidden enemy infantry in buildings.)

Use artillery to deal with pesky enemy artillery, anti-tank guns, and anti air guns so your normal units don't have to deal with them. Artillery is also very effective at destroying enemy dug in tanks and armored cars, and works just as good for enemy tanks and cars that's not dug in ether. (They are basically decent anti-tank guns when shooting at stuff not to far away.) if you know a pesky enemy unit is at a pesky place, but you cant see them due to fog of war, make the artillery aim at those specific coordinates by selecting the attack ground button. (looks like this: [+] )

Tanks and armored cars have Primary and Secondary ammunition. Primary ammo is for the main gun, while Secondary is for the machineguns, you can see the current ammunition supply in lower left when the unit is selected. (Is basically a bar.)

Never underestimate air superiority, it can help you tear your enemies to shreds. Just remember the enemy forces can send in air support as well. So if the enemy send in Ground attack planes, consider sending in Fighters to deal with them. Just don't send in your planes into Anti-Air guns, and if you do, try to destroy the enemy AA guns with artillery before sending them in.

Make use of the AA guns in your base during missions, take a truck and make them tow it, and place them at a better strategic location to assist you and cover your units against enemy ground attack planes better.

When dealing with enemy trenches, always try use your armored units to clean it form infantry, as infantry attacks causes heavy losses and is in general not worth it, while artillery is pretty much useless against it. Just remember that infantry in the trench may sometimes have anti tank rifles, so if you notice your lighter armored units starts losing health, ether quickly dig them in or pull them out to have them repaired. The unit AI prioritize shooting at infantry with ranged anti tank weapons, so hopefully your tank/armored car manage to deal with those pesky men before they manage to destroy your armored unit.

Always avoid attacking head on, try to attack from the side or behind whenever possible to neutralize the defensive boosts from their dug in stuff.

Always keep your core units alive, as they gain experience between missions, so its better to sacrifice a tank that's only used in that specific mission than a core one that gained valuable experience in combat that will be useful the entire game.

Hope this is to any help and make people find Blitzkrieg worth picking up!

Well, dang - a big plus one to you for that wealth of info. Gonna try a couple things to see if it cures my problem I've been having, where the game slows to glacial speed.

You can speed up and slow the game using the + and - key on the numpad, so maybe you accidentally slowed it down somewhat?

Nope. It's chronicled here: http://www.gog.com/forum/blitzkrieg_series/blitz_1_slows_to_a_crawl

Did you make sure to only run the game at one processor core? The game starts behaving very weird if you don't and then crash at random or during map switches/loadings. The game saves do save all craters on the map, blood spots, etc, but it should not have any performance issues with your specs.

Have you tried put 'Minimal FPS' to 0 in video options? Might be related to that considering how it slows down to snail phase for you. (Its by default set to 5 due to game was pretty demanding when it came out, or at least in some regions in eastern Europe.)

I'll give your earlier suggestions a try sometime... maybe this weekend. Gotta make room on the clock to devote a fair amount of time to it, since it takes some game-time for the problem to manifest; fortunately(?) I have a save or two just before the problem occurs. But I'll definitely give those a shot and post back with results. Thanks.

I've attempted to active the Motherland calls Mod, but I can't get it work, do you have suggestions on what I should do?

I have installed the game on a Corsair USB 3.0 128 GB Flash Voyager® GTX.
The game therefore resides in a "F:\GOG Games\Blitzkrieg\Blitzkrieg\" directory: no (x86) involved.
No problem playing it on a desktop PC running WinXP (Sp2), even if fielding a Core i7 CPU.
However, once I try to play it on my Win7-Core i5-laptop, the game inevitably crashes to desktop.
Tried to have it running "as Administrator", in "Compatibility Mode". No joy.
Any chance you can suggest a change to your command line to fit this peculiar situation?
Thanks for your time.

I'm using an 8-core AMD FX8350 using a stock GOG config setup. The game seems to run fine so far however I just started playing it and went through the tutorials successfully and only a little bit of the game. No crashes yet, but good to see all of the suggested fixes/workarounds and mods!

Links to mods are not working anymore, any chance some of you can re-upload them? Thanks!

I remember a (methinks) Czech site with various mods, but I can't find it anymore. I'm very much interested in a certain flora mod and a big (weapon) sounds mod. Thanks for your help!

"Stalingrad AI Logic" mod: link
"Tangram's European Winter Buildings" mod: link 1, link 2
"Achtung Panzer!" mod: link*

*Click "Other Links" in the top menu for various other Blitzkrieg resources.

o7

Now when you got the game up and running, a essential fix for Blitzkrieg 1 and its expansion is the Stalingrad Ai dll. Blitzkrieg have had many stand alone expansions, some licensed ones by other developers, one of them is 'Stalingrad'. It included a tweaked ailogic.dll that improved the Ai greatly, making sure it cant shoot thru objects such as houses and hit/see your units while you can't hit/see them.

Mods of Blitzkrieg-1 !
http://blitzkriegesp.blogspot.com/2012/12/asian-typhoon-mod.html
Welcome Blitzkrieg GZM v 9.20 !
http://union.4bb.ru/viewtopic.php?id=1270