From The Godfather:
And so it was Fanucci alone. Or Fannucci with some gunmen hired for a special job on a strictly cash basis. Which left Vito Corleone with another decision. The course his own life must take.
It was from this experience came his oft-repeated belief that every man has but one desteny.
I expected that only would have been instead of but there. Therefore I'd like to ask the reason for using but in that sentence.