I think the country you are a citizen in can still force you to come back to join the war if they want to. Either they already have such a law or they can create one when it's time. The country you would have a residency visa at would have to deport you.
Hardly ever it's going to happen for real, because it would imply expenses for the country where you are living to deport you in order to benefit solely your original country. You see mass deportation happening when it's of the interest of the nation where you are living, like it happens in USA.
USA only deport immigrants because they consider it to be benefical for themselves. They don't do this to help the countries those people came from. Another evidence of that are tyranical regimes such as the ones from North Korea, Cuba and Venezuela which may demand foreigner countries to send fugitives back to their original territories, although the refugees count with the protection and permission of foreigner countries to continue living there safely.
The same could be applied on a war scenario involving different nations around the globe.