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
When a part of a country leaves or breaks off, it is called secession. This can lead to the formation of a new independent state or country.
When one country takes over the land of another, it is called imperialism. Imperialism involves the extension of a country's power and influence through diplomacy or military force. This can result in the colonization or annexation of territories by the imperial power.
When you leave a country it is called emigration.When you arrive in a country it is called immigration.
A person who comes from another country is called an immigrant.
Mobilization
A country that is a 'friend' of another country is called an ally.
The movement of people from one country to settle in another country is called immigration.
The military service is called the service, because people are serving their country.
Mobilization
Those breaks are called mountain passes. They are natural routes that provide access for trade and military movement through mountain ranges.
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.