defect (verb) – stop being loyal to a country or group and leave them

defector (noun)  – a person who leaves a country or group of people because he or she doesn’t believe in it

defection (noun) – an act in which a person leaves a country or group because he does no longer believe in it

*There is another meaning of defect, which is fault.

Mickhail Baryshnikov

Here are some examples of how you can use it:

In their hunger for artistic freedom, a number of Russian artists defected Soviet Russia for the West.

Some books by defectors from North Korea have recently come out, which are in fact our only source of information about life in the mysterious Hermit Kingdom.

The defectors from the Labour Party established their own party, which was more centrist and rather successful.

Dancer Mickhail Baryshnikov’s defection was big news in 1974. In an interview he declared that life in Russia was a damn soap opera.