Concubines used to exist commonly before. Under what circumstances would they be allowed today?
In short: If there is a war with a non-muslim entity and the Caliph chooses to enslave the prisoners and grants a slave to a soldier then sexual relations can be had with those slaves and the slave can be sold to other owners who may similarly have sexual relations with them.
Non-muslim prisoners of war can be enslaved if the Caliph chooses that, he can also instead choose to ransom them, exchange them for Muslim prisoners or free them as a gesture of good will. He may base his actions on treaties that he has agreed to, such as the 1926 Slavery Convention: Islam lays stress on abiding by treaties unless the adversaries breaks them first.
These slaves can be distributed among the soldiers and also sold to others. It is permissible for the owners to have sexual relations with the women among the slaves if they are not married within the Islamic state. Their marriage before enslavement will be annulled if their husbands are not themselves captured, though there is a difference of opinion where some say that the marriage will not be annulled if the husband is also captured and enslaved.
If the slave is impregnated then she will become an Umme Walad, and may not be sold and will then become free on the owner's death.
Also refer to this answer by Medi1Saif