I didnt get the story (￣ε(#￣)Σ Please tell me (づ￣ ³￣)づ

This is a Hetalia doujinshi
The 'real' story is pretty funny :
http://www.mangago.zone/read-manga/hetalia/

There's also the anime version!

They mean Italy have an agree with the Adriatic sea,
and because of this Venice won't dissapear in the sea.
So that's why Chibitalia says Italy is the wife of Adria
And because Italy can't broke his vows, he's afraid that Germany can't love him with all his sternght.

I hope I can help youლ(´ڡ`ლ)

I get it. Thank you very much ヾ(☆▽☆)