the act of leaving a country, political party, etc. to go to another one

Over the years there were hundreds of defections to the West/defections from the East. 这些年来发生了数百起向西方／从东方叛逃的事件。

Recent changes in policy have resulted in large-scale defection from the party. 最近的政策变化导致了大规模的党员倒戈行为。