Tribute and Vassal States: Introducing New Diplomatic Options

This mod adds two new types of alliances: Tribute States and Vassal States. Each has its own unique method for establishing alliances and distinct alliance effects separate from other forms of alliances.

Do you find Civilization VI’s diplomacy system too rigid? Why do neighboring civilizations and city-states, often no larger than a small part of your empire, refuse to adopt flexible diplomatic strategies or yield to your superior power? Capturing AI cities is tedious and unrewarding, while widespread warfare damages your global reputation. Some countries might not even match the economy of one of your cities, yet they still stubbornly maintain poor relations with you. Shouldn’t your overwhelming economic superiority translate into diplomatic leverage?

We will teach these AIs a new world order.

Tribute States can be established by moving your unit near another civilization’s capital. A "Propose Tribute" button appears on the unit panel, allowing you to establish a tribute relationship by spending five turns’ worth of the target civilization’s gold income (requires more if your Diplomatic States is bad). There is also an option to offer a gift (specific effects are undecided, possibly acquiring their unique units).

Vassal States are formed through war. After meeting peace turn requirements post-war, capture their capital (not necessarily the original one) and use the newly added "Establish Vassal State" operation in the conquered city interface. Return the city and transform the defeated civilization into your vassal.

These systems also include interactions with city-states. When your unit approaches a city-state’s capital, the "Propose Tribute" button appears. You can offer a gift, costing five turns of the city-state’s gold income, to add an envoy to that city-state once per era. Engaging in combat against the suzerain of a city-state (never directly attacking the city-state itself), you gain one envoy in that city-state for every unit killed within its territory.

Alliance Effects for Tribute and Vassal States by Level:
(Upon establishment this two alliances, unlock vision within two tiles of all their cities)

Tribute State

Level 1: Trade routes to/from the Tribute State provide +4/+2 Gold; cities connected via trade route gain +1 Amenities (stacks across multiple tributaries). Your ally’s cities do not exert Loyalty pressure on yours, and vice versa (not working for now).

Level 2: Receive a free Eureka from your ally every 30 turns; cities connected via trade route gain +10% Great Person Points and Gold output, plus +1 Amenities (total of 2); stacks across multiple tributaries.

Level 3: Shared visibility; cities connected via trade route gain +10% Science and Culture output, plus +1 Amenities (total of 3); stacks across multiple tributaries.

Vassal State

Level 1: Trade routes to/from the Vassal State provide +4/+2 Gold; the Suzerain receives 50% of the Vassal’s Gold per turn. Your ally’s cities do not exert Loyalty pressure on yours, and vice versa.

Level 2: Units gain +5 Combat Strength when fighting enemies shared with the Vassal; cities connected via trade route gain +10% Great Person Points and Gold output (stacks across multiple vassals).

Level 3: Shared visibility; units receive one free promotion upon creation.

Revealing all the plots of the allied civilization upon forming the alliance and the gift-giving mechanic are planned but not fully implemented due to limited development time.

If this mod gets 25+ likes, we’ll add a one-time vision of all tiles revealed to the allied civilization after forming the alliance.
More likes may unlock further interactive features.

2025/5/4
Tribute cost now depends on diplomatic status with the AI. The gold cost multiplier is 5x when Friendly, 8x when Neutral, and up to 20x when Denounced.

2025/5/5
Added mutual loyalty pressure immunity between cities of allied civilizations in Tribute or Vassal States (Level 1 effect).(not working for now)

2025/5/5
Forming a Tribute or Vassal alliance grants one-time vision of all cities within two tiles of the allied civilization (does not grant shared vision).

2025/5/8
Updated diplomacy parameters: reduced the points required to level up alliances, and increased the alliance points from trade routes. Can be turned off in advanced options.

2025/5/8
Added a new post-alliance interaction (works with any alliance type): Hire Foreign Troops.Allows you to spend gold to hire the strongest land combat unit (by highest Melee, Ranged, or Bombard strength) that the other civilization can build in its capital.Maximum of three times per era. Hired units appear in your capital.This feature makes it easier to obtain other civilizations’ unique units for display.

2025/7/1
Wrote concept encyclopedia entries for Tribute and Vassalage systems.Improved the Vassal State establishment mechanics.Reduced the gold income multiplier required to establish a Tributary State with each new agreement (makes renewals easier).Fixed a conflict with Better Leader Icon (UI) mod.

2025/7/19

New interactive method added: by gifting another civilization a gift worth five times the base gold amount, you can gain one additional trade route capacity (once per era). If the gift is given to a city-state, you will also gain one envoy.
Changed the tribute system: each time you establish a tribute relationship, it now grants a 67% discount (was 50%) for the next tribute payment.

2025/8/11
Changed the gold contribution of vassal states to be produced in the capital city center (can benefit from city bonuses).