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
As here means because, and it heads the clause
As it is necessary to be free = because to be free is necessary
This clause is qualified by the clause in order to will, which is the complement of the adjective necessary—it expresses what freedom is necessary for.
The entire sentence refers to the Will of the People, which Rousseau talks about just before this. We may therefore paraphrase:
The people cannot will unless they are free. This causes another difficulty (which is at least as hard as the difficulty we just talked about): how can the people be free AND the government have authority at the same time?