An invasion is when an armed force enters another country or territory to attack, conquer, or take control of it. When one nation's army crosses the border into another nation without permission and with hostile intent, that's an invasion. History's most famous invasion might be D-Day in 1944, when Allied forces invaded Nazi-occupied France to help liberate Europe during World War II.

Invasions are serious acts of war that usually involve thousands or even millions of soldiers, along with tanks, aircraft, and ships. The Norman invasion of England in 1066 changed that country's language, culture, and government forever. Napoleon's disastrous invasion of Russia in 1812 showed how even powerful armies can fail when facing determined resistance and harsh conditions.

The word can also describe any unwelcome entry in large numbers. You might talk about an invasion of ants in your kitchen, or feel like your privacy was invaded when someone reads your diary without permission. A younger sibling might invade your room without knocking. These uses borrow the word's sense of unwanted intrusion, though they're obviously less serious than military invasions.

The key idea behind invasion is entering forcefully where you're not wanted or don't belong, whether it's an army crossing a border or mosquitoes invading your backyard cookout.