For those who haven't heard of what Victoria 3 is, it is a grand strategy game quite similar in design to the Civilization line of games. Like in Civilization, you control a nation, specifically one that begins in 1836 A.D., navigating challenges like hostile enemies attacking your borders or diplomatic disputes that you must solve through strategy. Among these diplomatic strategies is the ability to form a customs union with an ally, but what exactly does that mean?
Customs unions in Victoria 3
In Victoria 3, a customs union is an agreement made diplomatically between two nations wherein one of the nations is pulled underneath the umbrella of another nation's economy. This means that the nation being pulled in becomes part of the other nation's market and, therefore, must follow its rules and regulations. At first glance, this seems like a terrible deal for the nation being pulled in, but it can be a huge benefit in the event of a larger nation pulling a much smaller one and thus giving it an influx of economic opportunity it wouldn't otherwise have. It is also beneficial for the nation as it further bolsters its economy.
Rules about customs unions in Victoria 3
A customs union cannot simply be offered by any nation to another, as certain rules are in place to prevent this. Barring coercion or an owed favor, there is no real way to force another nation into a customs union. You also cannot strong-arm a nation into a customs union and cannot even offer a customs union to a nation that is hostile towards you. However, should you propose customs unions between two nations that are enemies but neutral or friendly towards you, they can both be pulled into your economy.