How were european slave traders able to acquire the millions of people captured and sold into slavery?

Africa. 400 years ago Africa was controlled by various warlords. Each warlord controlled swaths of territory, attacked other warlords peoples and sold them into slavery. Many times they would attack each other for the sole purpose of acquiring prisoners to be sold into slavery. The population was drastically reduced as a result. They only realized their predicament when the traders themselves invaded Africa with armies of mercenaries. For 300 years until the end of slavery, Africa was not free.