Ripstein, Arthur, Force and Freedom: Kant's Legal and Political Philosophy, Cambridge: Harvard University Press, 2009.

Excerpt:

“A political philosophy it is often thought of as an application of general moral principles to the factual circumstances that make political institutions necessary. For example, John Stuart Mill seeks to justify liberal institutions by showing that they will produce the best overall consequences, given a familiar  facts about human nature and circumstances; for John Locke, institutions can only be justified by showing that they are the results of individuals exercising their natural pre-political rights in response to the “inconvenience” of a state of nature.

Kant  might be expected to adopt a parallel strategy, applying the  categorical imperative to questions of political legitimacy, state power, punishment, or taxation, or perhaps viewing the states as a coordinating device that enables people to carry out their moral obligations more effectively. Alternatively, Kant might be expected to stand back from some questions, and recommends in difference to worldly matters of politics. Kant  Is often taken to understand morality exclusively in terms of the principles upon which a person acts. As such, it might be thought to depend contingently or not at all on the kind of society in which the agent found herself.

Such expectations quickly lead to disappointment.”

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