Negative Commandment #134
Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
Second Section – the Negative Commandments
134. It is a negative commandment "do not take back a divorced wife when she has already married another man."
as it states in Devarim (Deuteronomy) 24:4, "do not return to her first husband who divorced her after she has become ritually impure." If the first husband marries her again and has relations with her, he has transgressed this negative commandment, and he is coerced to divorce her. Even if she was not fully married but only betrothed, she is still forbidden to return to her first husband. But, if she had illicit relations after she was divorced, she is not considered remarried and is not forbidden to return to her first husband.
Included in this negative commandment is a woman who had illicit relations while married to the first husband which makes her forbidden to her husband, as it states in the verse above, "after she has become ritually impure" and here she became ritually impure. If she was forced and she is not the wife of a Cohain, then she is still permitted to her husband.
This applies in all places and at all times.