How did the slaves get kidnapped in the slave trade?

The vast majority of the slaves in the Atlantic Slave trade that supplied slaves to the Americas were bought by European slave traders from local African tribal leaders. The slaves came from two sources:

1) "Undesirables" that a nation wanted to get rid of. These might be criminals, heretics, the mentally ill, the indebted and any others that had fallen out of favour with the rulers.

2) The rest came from military conquests by African groups - many for the express purpose of capturing slaves for sale. The chief groups were the Oyo empire (Yoruba), Kong Empire, Kingdom of Benin, Kingdom of Fouta Djallon, Kingdom of Fouta Tooro, Kingdom of Koya, Kingdom of Khasso, Kingdom of Kaabu, Fante Confederacy, Ashanti Confederacy, and the kingdom of Dahomey. Most of the slaves sold into the Atlantic slave trade came from among those defeated in battles.

Europeans rarely entered the interior of Africa, due to fear of disease and moreover fierce African resistance.