Dumbledore is negotiating with the Council of Veela Enclaves about establishing an alliance. He wants their support in the war against Voldemort.
The Council agrees on the condition that Harry Potter marries their representative, moves to one of the enclaves, and has at least two daughters with her within a few years.
Dumbledore agrees, but manages to convince the Council to keep this a secret from Harry, in order not to pressure him into a political marriage.
The Council has no problem with this, confident in the abilities of their representatives.
And so Council Agent Fleur Delacour moves to Britain with a mission. Seduce and marry Harry Potter.