Reuters recently reported that the United States would sail a third ship near the Chinese-built artificial islands in the South China Sea. It would be difficult for the Obama administration to telegraph more clearly it has absolutely no interest in China's new islands. Or, for that matter, Russia's new Ukrainian possessions, or Iran's bludgeoning Syrian rebels into submission. To the president, these are unrelated drips and drops.

Jakub J. Grygiel and A. Wess Mitchell's new book, The Unquiet Frontier, links them together. It is the intellectual backbone to every bumper-sticker critique about the "weakness" of Obama's foreign policy. Their book examines America's inaction in the face of aggressive policies by revisionist states like Russia, argues persuasively that such inaction is a disaster. Apathy towards Ukraine or Syria or the South China Sea not only cedes local stakes to the revisionist power and encourages future aggressions, but also entails a whole host of unpleasant second-order effects. Without American security, U.S. allies in the Rimlands – their preferred word for smaller states like Poland and Vietnam close to the larger revisionists – may act unpredictably, even aggressively, to stay safe.

It is a densely argued book, and quite thorough. The authors specialize in modern Europe, the classical European balance of power, and also antiquity; some of the references are fabulous, and occasionally hysterical. I am quite sure no other book on American foreign policy has ever referenced the scrappy medieval kingdom of Burgundy. Elsewhere, they illustrate the balancing effect of alliances with the example of Armenia, which the Romans "famously" used to check the Persian empire's expansion into Syria. Anybody who thinks Rome's Armenia policy is famous is someone who should probably be writing foreign policy books professionally.

Their book is also timely. Grygiel and Mitchell are arguing that America's commitment to its latter-day allies like Poland and Lithuania does indeed matter; that they are not, pace Donald Trump, a bottomless cost center and are not, pace Obama, less important than gauzy bargains with their predators. These alliances do have a benefit, and are worth defending.

However, they could have spent more time on the relationship of these security commitments and the norms of the world order that most directly benefit America, like global free trade. The benefits of these alliances, they argue, are worth the price of commitment, for in the case of a conflict they offer the U.S. capabilities it wouldn't have otherwise. But the core of the argument is that the U.S. doesn't have to fight these fights; that it wouldn't need Rimland allies to balance Russia if it granted Russia its traditional sphere of influence over the Rimland. And that influence, doves would argue, does not imperil the fundamental tenets of the liberal international system that make America prosperous and free.

It is this argument that needs winning. Grygiel and Mitchell address it in a few pages, but it deserves more. The relation of the Rimlands to the American prosperity is at the core of the debate over the foreign policy of Obama and Trump and Cruz and Clinton. Does bandwagoning behavior by Taiwan imperil the dollar's status as the world reserve currency? That's what the presidential candidates are fighting about in their talking points and bumper stickers. Grygiel and Mitchell have done us a great service by filling in the gaps.

Andrew L. Peek was a U.S. Army intelligence officer and is currently a professor at Claremont McKenna College.