Which phrase or word does “as” belong to?

As, in order to will, it is necessary to be free, a difficulty no less great than the former arises: that of preserving at once the public liberty and the authority of government.
_{Source: Rousseau, A Discourse on Political Economy}

Which phrase or word does “as” belong to?

As Damkerng T. pretty much gave you in the comment, "as" in this case means "like" (prep. 2 b) or "since" (conj. 3).

"Since it is necessary to be free in order to will..."