An invasion is when one country or group attacks and takes control of another country or area.
Imagine two countries that don't get along. One day, one of them decides to send soldiers into the other's territory without permission. This can happen with armies or even a small group of people trying to take over. In some cases, an invasion can be a surprise attack where the defending country doesn't know what's coming until it's too late.
How common is "invade"?
Word invade is considered uncommon in modern English. It has a balanced usage among all categories: speech, fiction, newspapers and academic texts.
Definitions
verb
(transitive) To move into.
Example: Under some circumstances police are allowed to invade a person's privacy.
(transitive) To enter by force, usually in order to conquer.
Example: Argentinian troops invaded the Falkland Islands in 1982.
(transitive) To infest or overrun.
Example: The picnic was invaded by ants.
To attack; to infringe; to encroach on; to violate.
Example: The king invaded the rights of the people.
To make an unwelcome or uninvited visit or appearance, usually with an intent to cause trouble or some other unpleasant situation.