Attack on One is an Attack on All

[Teutonic Germanica]

In order to maintain Peace in the World we must set up an Alliance that when One member of the Alliance then it shall be viewed as an Attack on all of the members of the Alliance. A Region shall Join the Alliance and all of the Countries that are within that Region that joined that Alliance are considered as automatic members of the Alliance, the region shall elect a delegate to this alliance to represent their regions to discuss the topic of what to do if a rogue Nation invades another Nation, a rogue region invading another region or raiders invading a region, etcetera. This Alliance shall be strictly used for Defensive Purposes and to keep Peace around the World.

[The Atlae Isles]

In order to maintain Peace in the World we must set up an Alliance that when One member of the Alliance then it shall be viewed as an Attack on all of the members of the Alliance. A Region shall Join the Alliance and all of the Countries that are within that Region that joined that Alliance are considered as automatic members of the Alliance, the region shall elect a delegate to this alliance to represent their regions to discuss the topic of what to do if a rogue Nation invades another Nation, a rogue region invading another region or raiders invading a region, etcetera. This Alliance shall be strictly used for Defensive Purposes and to keep Peace around the World.

You can't force nations to join an Alliance. Such a proposal would be illegal by metagaming.

But you can make a region called "the Alliance" if there isn't one already, and make embassies with other regions. But this is no place for the WA.

[Volitopia]

Sounds a bit like a defender alliance + large and powerful group.
I recommend starting a "home region" (to hold meetings) for your alliance and growing it out from there. You don't need people to join the region. It'll probably some people just looking out for each other, which sounds nice but it's hard to keep track of even one nation for a while.
If you want to prevent raiders/invasions then join/start some defender region!