A LOT of countries on the planet talk about annexing their former territories, like Orbans Hungary. Others have actually done it (Armenia- Azerbaijan).
What do you want to do about it? Start a world war with them just in case to provent them from doing it (further)? Bombing them in the name of peace?
"Preventive war is like committing suicide out of fear of death."
Otto von Bismarck
The best defense is to have a military strong enough they won't dare attack.