The thesis of this book is that the post-Cold War foreign policy of the United States of America has been a disaster and its enduring appeal and continuing application is due to an entrenched and protected self-interested foreign policy elite that dominates domestic discourse. Walt identifies their failed foreign policy as liberal hegemony which he defines as an overly ambitious foreign policy seeking to influence nations to adopt a suite of liberal reforms - democracy, market economics, property rights, etc. - often through diplomacy but increasingly through threats and military force.
It's a persuasive argument and Walt marshals the familiar failures of the policy: Vietnam, Iraq, Afghanistan, Libya, NATO expansion (leading to an increasingly paranoid and bellicose Russia), the numerous adventures in Central America, as well as many others. These policies all lead to America being less secure and less prosperous. Equally persuasive is Walt's argument for why, despite such obvious failures, liberal hegemony remains the dominant foreign policy of the United States. It remains because America's foreign policy elite won't let it die. They're able to sell their vision largely by means of threat inflation, benefit exaggeration, and cost minimization. Further, they avoid accountability because they are a tightly knit community and the unparalleled riches and security of the United States affords them a robust bufferzone of failure. Walt concludes his book with a short chapter on his preferred foreign policy: offshore balancing. A policy of offshore balancing calls for a restrained commitment to large-scale social engineering abroad, a commitment to maintaining a minimal military footprint in foreign countries (outside of a few strategic zones), and an emphasis on diplomacy in international relations while allowing local powers to balance any potential antagonistic foreign hegemons.
I've only read a few books on foreign policy and have hardly given it any thought so obviously I can't comment on the wisdom of Walt's arguments but they do make intuitive sense to me and I'd like to see us move more towards his vision.