In the U.S., naturalized citizens are routinely stripped of their citizenship if they are proven to have committed fraud during the naturalization process (several cases of former concentration camp guards come to mind). To become a U.S. citizen, an applicant also has to swear loyalty to the United States, and renounce allegiance to any foreign country or ruler. If they are afterwards found to have violated that oath, their citizenship can be taken away. So if a naturalized U.S. citizen, for example, travels to an enemy country and denounces the United States, their citizenship could be lawfully revoked (although I don't know of any cases where that has happened).
from the article: Conditional citizenship