If you live in the states and the person you want to marry has no right to, then getting a fiance(e) visa may allow you to reunite. The official term for it is a K-1 non-immigrant visa.
You can only do it if you are a U.S. citizen. So if you are not, you need to see if you can get naturalization first.
Are they really your fiance(e)?
The immigration authorities will want to know if the relationship is for real when considering the application. You need to prove your love for each other, not by kissing in front of an immigration official, but through as much evidence as possible such as old photos and cute texts. Any paperwork with both your names on it will be a massive help. For example, submit a copy if you rented an apartment together when you lived in their country, and the contract features both names.
Are you sure you are not already married?
If one of you was previously married to someone else, make sure you have up-to-date paperwork showing you are either divorced or widowed. If that marriage was registered in more than one country, make sure you have paperwork showing it is over for all of them. You may need to get it translated by an approved translator. You might also need various consular stamps to make things official.
Are you really going to marry them?
If the marriage does not happen within 90 days of them landing on U.S. soil on their visa, the visa will die with the romantic aspirations.
Like any visa, the authorities often reject K-1 applications. Getting help to navigate the country’s strict immigration laws could make all the difference between success and singledom.