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All in all 15 enemy regimes left that status as result of either negotiations, or by actions that the US has little to do with - so hostile interventions aren't more effective than these. Literally just waiting the enemy to collapse might be more effective than... let's say bombing them.
As the book puts it:
Since most U.S. enemies eventually leave that status (in 2008, only five states were still enemies), this calls into question the utility of hostile intervention.
And again they drive their point home, US foreign policy is means driven. They have tools to deal with situations and they apply them, no matter of the cost:
When to these considerations are added the enormous potential human costs of those interventions for both target states (combat deaths, civilian casualties, deliberate massacres) and the U.S., not to mention the budgetary implications, such operations appear even more means-driven than their client counterparts.

The book talks about the future of client-state imperialism, the possibilities if it could stop. Their conclusion is unlikely. Since the publication of the book things doesn't seem to change, if we look closer, we can see the behavioural patterns.
I highly recommend this book to anyone who wants to see the US foreign policy in a different light, and to get some insight how it works. And how it doesn't.