Easy.
According to Islam, it is okay to kill and plunder unbelievers so long as the Prophet gets his share. Likewise, your captives belong to you as slaves - and female slaves are yours to rape, as the women of infidels.
Loot, sex slaves, and Paradise if you are killed in battle. Sounds pretty attractive to certain sorts.
Mad Max Merkel has destroyed her country. Too late to shut the gates after you’ve invited the barbarians to dinner. Germany is lost.