Rubuta ci gaba don wannan sakin layi - Mutanen Espanya conquistadores karkashin jagorancin Hernán Cortés sun ci babban birnin Aztec na Tenochtitlan
(a wurin da yanzu ake kira Mexico City) a shekara ta 1521, kuma 'yan mishan na Franciscan suka bi bayansa ba da daɗewa ba a shekara ta 1524. Sahagún ba ya cikin wannan rukuni na farko na 'yan uwa goma sha biyu, waɗanda suka isa New Spain a 1524. Sahagún ya yi bayani, a cikin Mutanen Espanya da kuma Nahúatl, game da jayayya da waɗannan 'yan Franciscan suka yi a Tenochtitlan ba da daɗewa ba bayan isowarsu, a shekara ta 1564, don ya ba da misali ga masu wa'azi na gaba. Saboda iliminsa da kuma matsayinsa na addini, an zaɓi Sahagún a shekara ta 1529 ya shiga aikin wa'azi a New Spain. Zai yi shekara 61 a wurin.