double-war can be used to force a non-HRE country to join HRE

[DragonWhite]

After version 1.23, decisions that move your capital now removes that province from the Holy Roman Empire if you are not a member state and the capital is being moved into the Empire. This will also make the Empire dislike you as usual, and reduce the Imperial Authority. (From ChangeLog)

However, you can still use a double-war to force a non-HRE country to join HRE.

For example， PEST is the current capital of Hungary.

Then the emperor Austria conquer PEST and make it a HRE province.

Then return it to Hungary. The current capital of Hungary is SZABOLCS.

Then Austria declare a second war and conquer SZABOLCS. PEST become the capital of Hungary again.

Now Hungary is a member of HRE.

 

[Dominion]

I mean... yes? Obviously?

You forcing them to move their capital means they didn't decide to move it. You made that decision for them.

It's been a gimmick for as long as I can remember. Used to turn France into an HRE prince ever since I showed the game to friends in Nice.

 

[Gratak]

Pretty obvious indeed. Ask the pope (Rome -> Avignon/Province and back again)

 

[Rikissa]

This isn't always a good thing because Hungary/Poland and especially France might start eating HRE minors if you add them