When the military forces of a foreign nation "breaks" into another nation for the purposes of controlling that nation or for the purpose of occupation or annexation, it's called a war of invasion. There are many factors involved with the question asked, and therefore more information is required to give a more detailed answer.
The military service is called the service, because people are serving their country.
A territory under the control of another country is called a colony.
It is called an invasion.
There was no country called "Vietnam" during the war. There was a country called North Vietnam and another country called South Vietnam, which one are you asking about?
Self-Autonmy
Mobilization
A country that is a 'friend' of another country is called an ally.
A piece of country rock that breaks off and becomes entrapped in a body of magma is called a xenolith.
The military service is called the service, because people are serving their country.
Mobilization
It is called emigration when you leave one country and immigration when you arrive in another country.
its called nationalism or, it could be colonization.
When a company hires people in Another Country to do work for them, it is called outsourcing.
When a company hires people in Another Country to do work for them, it is called outsourcing.
A person who leaves a country to go live permanently in another is called an emmigrant,
It's called conquering
That is called an 'Alliance' or a 'defence pact'