The Malus, an alien that's purely evil, needs the civil war re-enactments to become authentic so it can feed off the psychic energy of dying and embattled men and fully revive. Not if the Doctor can derail things, of course.
Did You Know?
While filming the scene where The Doctor, Wolsey and Will Chandler arrive in a horse-drawn cart at the false lych gate, the horse was reluctant to approach the lych gate because his mare was grazing in a field nearby. The mare was brought inside the churchyard - out of shot - to encourage the animal to behave properly. Unfortunately, the horse became so excited that, on the take, it proceeded to follow the actors through the lych gate, destroying it in the process. Luckily, the shot up to that point could still be salvaged for use in the finished episode. See more
Pains me to say it but I'm sorry I ever doubted you.
Well, we all learn by our mistakes.