It's bad enough she broke your heart, you should at least be able to recover the fortune you spent on the ring, right? The law agrees with you in most states, but litigation is expensive. How can you go about getting that engagement ring back with out it turning into a big fiasco?
If you are not already in this situation, but are just having cold feet about getting engaged, just remember this: Don't give her the ring on her birthday, or a holiday, or some other special occasion. Then it's a gift and it's hers regardless. But if you avoid these occasions then many states view engagement rings as a conditional gift whereby if the receiver doesn't follow through on the condition of marriage, the ring returns to the giver. If you broke up with her, on the other hand, or if she broke up with you because you cheated on her you can pretty much forget about it.
Ultimately, the best thing to do is ask for it back. If she won't talk to you then find a go between. Don't mention legal stuff. No matter how hurt you are, it is cheaper to just ask nicely and hope for the best. Unfortunately, if she refuses to give it back then you are probably headed for small claims court.
You will want to check the law in your state to see if engagement rings are considered a conditional gift, an implied conditional gift, or an unconditional gift.
Some have chosen to include the matter of the engagement ring in a prenuptial agreement. Thus, even if the fiance follows through on the marriage and then leaves, the ring will still return to the giver. Many men have particularly made this a point when the ring being used is a family heirloom. Others find the idea of prenup offensive, considering it an easy way out of marriage financially. Whatever you decide to do, keep in mind what state you are currently living in, and if you move, find out about the laws in your new home state.
Honestly, you can't return the ring anyway. You will have to pawn it for a fraction of what you paid. At some point you may decide it isn't worth court costs. But if you went all out, or if the ring has sentimental value (it was you mom's, for example) then you may decide to fight for it at any cost.