If you just want a divorce, depending on where you live, you can either separate from your spouse for a period of time, or tell the court that you have irreconcilable differences or your marriage is irretrievably broken. In most cases, you don't have to prove to the court that your marriage is broken, and you don't have to cast blame. If you want an annulment, however, you lose this no-fault advantage. You must prove that something was inherently wrong with your marriage even before you said, "I do."
Void vs. Voidable Marriages
Annulment grounds fall into two categories: some apply to voidable marriages, and others apply to void marriages. A void marriage is one that was illegal in some way from the beginning. For example, if either you or your spouse were already married, marrying again would be bigamy. Bigamy is illegal, so your marriage is void. It's not actually necessary to legally annul these marriages, although it may be a good idea just to tie up loose ends. Voidable marriages are those that appear OK on the surface but involve some factor that gives either spouse grounds to contend that the marriage shouldn't have occurred, or that either spouse would not have married had he been aware of the problem. Examples include impotence on the part of one spouse, or that one spouse lacked the mental capacity to understand the commitment he was making, either for mental health reasons or due to the influence of drugs or alcohol. States set their own annulment rules, so a marriage that is void in one state may be voidable in another.
Essence of the Marriage
Annulment grounds that go to the very essence of a marriage usually involve fraud. Marriage is a lifelong commitment that requires spouses to place the utmost trust in each other. If one spouse withholds a circumstance or truth that would affect the other's willingness to commit, this goes to the essence of the marriage – it defies what marriage is supposed to be. A classic example is a spouse marrying solely because she wants a green card. She has no intention of making a lifelong pact with her spouse – she just wants to immigrate. Another example would be someone saying that she wants children, but after the marriage, she takes great pains not to conceive. Grounds that go to the essence of the marriage must negate the concept of marriage. They don't include things like concealing a bad habit, such as drinking, smoking or gambling.
If Your Spouse Contests
If your spouse contests the annulment, a typical defense is proving that you resumed marital relations and continued living together after you learned the truth. For example, if your spouse only wanted a green card, but you tried to make the marriage work for six months after you found out about this, you might have to seek a divorce instead. Your spouse can also argue that the fraud you're alleging did not go to the essence of your marriage. For example, she might state that you said you didn't want children, so concealing her feelings on the subject did not necessarily violate trust.