Isn’t an annulment the same as a divorce?
No. A divorce is a legal decree that a valid, legal marriage has been done away with. In the eyes of the state, a marriage can be real and still be dissolved. The Church, however, recognizes that before God a valid marriage between two baptized people can never be dissolved, that spouses are bound to one another as long as they live. An annulment, therefore, is not a decree that this bond has been dissolved but that, on investigation, no marriage bond ever existed.
When two people seek to be united in Christian marriage, certain realities must be present in order for that union to take effect. For example, if one partner is being forced into the marriage, or if one does not intend to be faithful or to be open to children, he or she is not entering what God considers a marriage. Therefore the marriage is not valid, no matter what the state may think about it. (There are other conditions, but I mention these for starters.)
Imagine that a couple went through a wedding ceremony, but the bride was being forced to marry the groom. Even though it looked like a wedding, there was no valid marriage. Since there was never a marriage to begin with, they are not bound to each other. Their “marriage” could be declared null—found by the Church to have no existence in the eyes of God. Since God has not joined them together, they are free to marry other people. Jesus spoke of this in the Gospel of Matthew, when he forbade divorce and remarriage, “unless the [first] marriage is unlawful”—that is, null (Matt. 19:9, NAB).
So an annulment does not end a real marriage but declares that there never was a sacramental marriage to begin with. The Church goes through a long investigation to determine if the marriage was validly contracted. If it was, then even if the marriage turned sour years later, the Church cannot dissolve that. (The couple may separate if necessary, such as in the case of abuse, and even may obtain a civil divorce, but neither is free to remarry.) When a valid marriage has taken place between two baptized persons, only death can sever that bond.