Divorce is not the only way that marriage can end. A marriage can be severed by annulment if the court determines that it was not legally valid. In the law's eyes, once a marriage has been annulled, it never existed at all (because it was not legally formed).
There are a variety of reasons for why a marriage may be invalid. Immediate disqualifiers are incest and bigamy. If two blood relatives marry each other, the court will nullify the marriage. Similarly, a marriage will be nullified if one spouse was already legally married to a third party. Additionally, a marriage can be annulled on the basis of age. If one party is a minor (under 18 years), he or she cannot give consent, and thus the marriage contract is invalid. Marriage can also be annulled if it is found that one party was forced into the marriage through force or threats. Finally, a marriage may be annulled based on fraud. For example, if one spouse entered the marriage only to receive a green card, the marriage would be nullified. Marriages in pursuit of a goal rather than love and companionship will be deemed fraudulent.
Nonetheless, fraud can be very difficult to prove as a reason for annulment. The same goes for reasons such as pressure and duress. If you are facing the prospect of annulment, be sure to consult a legal professional to learn how you to defend your marriage or mount an effective challenge.