If you're not Catholic, but plan to marry a Catholic, you might be asked to go through the annulment process. This seems odd to most non-Catholics because neither person from the first union is Catholic. Therefore, why should the Catholic Church investigate this marriage?
The Catholic Church presumes the validity of any marriage between two people who are free to marry at the time of their wedding. (They must have no previous marriages.) Basically, if the non-Catholic religious community of either spouse recognized the marriage, so does the Catholic Church. Since marriage, as God created it, is permanent, then the Catholic Church must also investigate these marriages. Because the non-Catholic wishes to marry a Catholic, the Church's law applies to the proposed marriage, since canon law still binds the Catholic whom the non-Catholic wishes to marry.
In short, the Catholic Church believes her teachings concerning the essence and the properties of marriage bind all people, regardless of whether they are Catholic, as part of God's natural law.
Are there options for working with previous marriages other than the annulment process?
Yes. For a person who was either Catholic or married to a Catholic, and did not marry according to the canonical form of marriage (in front of a Catholic priest or deacon with two witnesses), and if the Catholic Church's permission was not obtained for this marriage (called a "dispensation from canonical form"), then the Church could process this case as a "Lack of Form." The Church calls this an administrative process.
In this case, the individual must prove that one of the former spouses was Catholic, that the couple attempted marriage outside of the Catholic form without first obtaining the proper dispensation, and that the marriage is now irreparable. The individual must also establish that this marriage was never subsequently convalidated (commonly, and mistakenly, referred to as "blessed" by the Church.) Most marriage tribunals accept as sufficient proof of these circumstances the Catholic's baptismal record, a copy of the marriage license, and the couple's divorce decree. Nevertheless, depending upon particular circumstances, more evidence may be necessary.
For a person previously married to someone with a prior marriage, provided the Church had not dealt with the prior marriage, a documentary process exists. This is called ligamen, or prior bond. In most cases, one simply documents the prior marriage of the individual with whom one attempted marriage. The local tribunal advises the couple concerning what documents they require and guides them through the process.
If one of the spouses was not baptized during the first marriage, and the lack of baptism can be proven (provided the person applying for this process did not cause the marital breakdown), then a "Privilege of the Faith" case (or "Petrine Privilege" case) can be sent to the Holy See. If the Holy See approves, the non-sacramental marriage may then be dissolved in favor of a new marriage.
If neither of the spouses was baptized during their marriage, and now one of the spouse's wishes to become baptized and marry a Catholic, provided one can prove the non-baptism of each former spouse, a Pauline Privilege is possible. In this situation, the diocesan bishop or his lawful representative, having established the non-baptized status of both parties, allows the non-sacramental partnership to be dissolved in favor of the new marriage. Of course, the spouse desiring baptism and the new marriage must first receive baptism.
A Basic Rule
If you are trying to determine whether you need an annulment, these explanations may be helpful. In any case, keep in mind one basic rule as you approach the process: If either you or your intended attempted a previous marriage, be sure to tell your priest. Before you attempt another marriage, the Church must address the previous marriage in some form or another, either by a documentary case, a privilege case, or a formal annulment process.