As we all know, conflict in Victoria games is most often found either at the diplomatic table or among the various trade alliances. In Victoria 3, those are the customs Unions, as the developers explain in their latest dev diary.
Within the scope of Victoria 3, a customs union is a bilateral agreement between two nations where one nation agrees to subject its national market to another and to be absorbed into an economic union, the devs start off their explanation in the latest dev diary.
This means that it’s a diplomatic action, that is, something another nation has to agree to as a customs union alone is not something that is normally able to be forced upon another nation. A customs union is not limited to only two partners, other nations may make themselves subject to the senior partner alongside other participant junior nations but a customs union can have only one senior member in control of the union.
Slight variance to the above statement is possible, custom unions can be forced through by utilizing obligations by the previously owed country. This will only allow the diplomatic action to take place despite the nation’s disposition to say no, it should not be able to supersede any other limitations possible in the game.
Customs unions themselves cannot be forced through war but they can be forced as a result of other diplomatic entanglements. Certain subject relationships come with the implicit expectations of joining the shared market of their overlord but they also come with other strings attached.
The two nations, that make up a union, must have neutral or positive relations to conduct this action. A nation that sees you in an antagonistic light as you are attempting to wipe them off the map isn’t going to give you access to its market willingly.
Two nations that are trying to wipe each other off the map can both be junior partners in the same customs union led by another nation, just expect all the market disruption that would come with such chaotic developments if it's your frustration to handle as the overseeing power.