How To Form Siam in EU4 (Step-by-Step Guide)

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You can form Siam if your primary culture is in the Tai culture group. Of all the eligible nations, the two best contenders for this are Ayutthaya and Lan Xang.

However, Ayutthaya has an additional tech requirement for forming Siam. They’ll have to wait at least 200 years just for that tech.

Lan Xang only needs the target provinces, and they can get those easily within 10–20 years.



The “Form Siamese Nation” decision requires that you:

  • Own these core provinces:
    • Ayutthaya
    • Sukhothai
    • Chiang Mai
  • Own at least 20 core provinces in the Indochina region
  • Must not be a subject of another nation, except as a tributary
  • Must be at peace
  • (For Ayutthaya) Must have at least Admin Tech 20

Note: Admin Tech 20 becomes feasibly available around 1661.

Indochina and the Three Required Cores Marked in X / EU4
Indochina and the Three Required Cores Marked in X



#1: As Ayutthaya

You’re the most powerful in the region. Your capital, Ayutthaya, is the first of the required provinces, and you have a vassal who owns Sukhothai.

Getting Chiang Mai and the rest of Indochina is just inevitable.

Ayutthaya’s Achilles’ heel is their extra requirement — Admin Tech 20.

You’ll have to wait at least 200 years before you can reach that tech level. It’s not hard, but it may become boring.


#2: As Lan Xang

Lan Xang is the second largest Tai nation after Ayutthaya, who’ll be your final obstacle in trying to form Siam.

Before you go after them, you’ll want to expand your country first.

Conquering or subjugating Lan Na and Khmer should be your priorities.

Step 1: Conquering Lan Na

Lan Na owns Chiang Mai, one of the required provinces for Siam. They’re also geopolitically isolated at the start.

Fabricate a claim on Nan province ASAP.

Once you’ve got the claim, immediately declare war on them.

Occupy Nan province and then lay siege to Chiang Mai, their capital.

After it falls, continue on to occupy their remaining provinces until they finally surrender.

Sue for peace and fully annex them. Don’t worry about the diplo mana costs of unjustified demands. You can recoup your lost mana by attacking Khmer.

You’ll be able to complete your “Subjugate Lan Na” mission after annexing them. This rewards you with a “Subjugation” CB on Ayutthaya. Don’t claim this reward yet until you’re ready to attack Ayutthaya.

Lastly, keep the fort in Chiang Mai intact. Its position and its jungle terrain makes it great for impeding your enemies.

Step 2: Humiliating Khmer

You want to be opportunistic when dealing with Khmer. They’re about as big as you, but they suffer from religious instability.

This makes them an excellent target for the “Humiliate Rival” CB. You can farm them for mana by using the CB’s “Show Strength” peace term. You’ll gain 100 mana for all three MP pools every time you do this.

Ayutthaya usually attacks Khmer within a year or two from the start. That’s your first window of opportunity.

Raid Khmer and use the mana you’ve gained to develop your country and make it more powerful.

Later, when you’ve conquered Ayutthaya, you can stop humiliating Khmer and just conquer them outright.

Step 3: Conquering Ayutthaya

Your final target will be tougher to beat. If necessary, hire mercenaries or keep expanding your nation across Burma.

Once you’re ready, claim the reward from your “Subjugate Lan Na” mission.

Declare war on Ayutthaya using your new “Subjugation” CB.

Remember to set the “Defensive Edict” on your provinces that have forts, particularly your capital. This adds +33% defensive to your forts, which can delay any siege.

Most of the time, Ayutthaya will be allied to Pegu. If this is the case, go for Pegu’s capital first. You can force them to drop out of the war if you can take their capital.

For Ayutthaya, their capital is in a farmland province, which provides no defensive bonus. Try to avoid fighting their army and just prioritize sieging their capital.

When you’ve finally prevailed, annex the provinces of Sukhothai and Ayutthaya during the peace negotiations.

You can force Ayutthaya to become your vassal if you still have enough war score to do so. If not, you should take as many provinces as you can to weaken them.

After all this is done, you’ll have all the required provinces to form Siam.

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Louie Nelson Zafico

As a frustrated otter who dreams of getting published, Louie instead wastes his life cuddling his cats. He spent his childhood playing Suikoden, grew up with Total War, and matured (somewhat) with EU4. He hopes to someday find a geopolitical JRPG with the 4X systems of a Paradox game.