Parshas Ki Seitzei: Meshech Chochma - Exchanging Prisoners Of War
In the Parsha of Yifas To'ar when it becomes permissible to keep and marry a captured women, the torah describes how we go out to war, "UNisano Hashem Elokecha B'Yadecha V'Shavisa Shivyo", Hashem gives the enemy in your hands and you capture a prisoner (Ki Seitzei 21:10). Why does the torah mention that the enemy fell into our hands?
Rav Meir Simcha HaKohen MiDvinsk in the Meshech Chochma says that this is a condition to keeping a Yifas Toar. In war prisoners are captured and then when the war ends they are exchanged and repatriated. In the wars of Bnei Yisroel, if both sides captured prisoners then it would be forbidden to keep a Yifas Toar. Why should Jewish soldiers remain captive so that another Jew can keep his prize captive? Therefore the torah tells us that this parsha only applies when Hashem puts the enemy in our hands and not vice versa.