Still Another Day in the Life - S7-E22
Ben Veloric: I guess I just wanted to find out of she loved me for myself.
Harry Stone: Now I guess you know, all she cares about is money.
Ben Veloric: Yeah. So, Debbie, are we back on?
Debbie Rollins: You mean you don't care that I'm shallow and materialistic?
Ben Veloric: Heck no, as long as you don't chunk up.
Answer: Being declared legally dead is called "death in absentia", meaning there is no evidence of death (i.e. a body), but the individual is presumed dead. This can happen to anyone, not just MIA soldiers. If a spouse petitions the courts to grant a divorce on the presumption of death and all criteria are meet, they are legally divorced and free to remarry. The return of the presumed dead would not change the divorce ruling. However, if a spouse does not go through the legal steps prior to remarrying, then the return of the presumed dead spouse would nullify and void the 2nd marriage immediately (and the remarried spouse could face bigamy charges).
Bishop73