The best overview of America's current world-stage challenges I've come across

American Enterprise Institute scholar Gary Schmitt has a new book out, called Rise of the Revisionists: Russia, China and Iran.  He has written the introduction, and there are essays by other AEI world-affairs specialists on each of the countries mentioned in the title:

Frederick Kagan's "Russia: The Kremlin's Many Revisions," Dan Blumenthal's "China: The Imperial Legacy," and Reuel Marc Gerecht's "Iran: The Shi'ite Imperial Power"

There's a concluding chapter by Walter Russell Mead that sums up how America should engage each of these spheres:

In the concluding essay, Walter Russell Mead explains why international relations realists are inclined to define a state's behavior narrowly. As a result, they do not provide an adequate road map for policymakers to use in developing strategies to confront that behavior. "Thucydides was no realist in the modern, American, and academic sense of that term." Today's realism "is a weak and denatured creature, compared to the complex vision of Thucydidean realism, and the costs to analytic coherence are serious." 

No, Thucydides' notion of realism went like this:

He also has been called the father of the school of political realism, which views the political behavior of individuals and the subsequent outcomes of relations between states as ultimately mediated by and constructed upon the emotions of fear and self-interest.^[3] His text is still studied at universities and military colleges worldwide.^[4] The Melian dialogue is regarded as a seminal work of international relations theory, while his version of Pericles' Funeral Oration is widely studied by political theorists, historians, and students of the classics.

An abridged version of Schmitt's introduction appears today at AEI, and it's important reading. Here's his - and Mead's - application of Thucydidean realism to the 21st century scenario of the three revisionist powers:

In Mead's account, in the world of Thucydides, peoples and leaders are moved by a complex mix of interests, fate, and passions, and "no concept could be less congenial" to the Father of History "than the idea that domestic politics and regime type are largely irrelevant to the study of international relations"—which holds true for both autocratic and liberal regimes. In short, it pays to know, in depth, what is driving a state and its leaders; to understand that those drivers cannot be divorced from a country's internal governance; and to realize that, even with such an understanding, unknown and uncontrollable factors will still intercede to shape and limit any strategy.

The way these powers are acting is straight out of the way Athens regarded Melos in Thucydides' telling of it:

When a regime's character is factored in, tensions appear virtually inevitable. China, Iran, and Russia all assert a civilizational challenge to the Western liberal democratic order. It is difficult to know how deeply the three countries' general populations hold their leaders' views, but for the leadership in each, ideology is certainly an important source of legitimacy for their non-liberal rule at home. 

Coexistence with prosperous, relatively powerful democratic neighbors, whose own relations are largely based on the trust and norms that come from similarity of rule, is a circle hard to square. Even Iran, whose neighborhood is hardly filled with liberal democratic states, must continually strive to keep Iraq, Lebanon, and Syria in a state of chaos, lest a more liberal, majoritarian Shi'ite state emerge and threaten the Islamic Republic's claims to be the only legitimate form of rule for its sect. And the notion that a nation carries a special civilizational role becomes even more important for the leadership to sustain when their ability to meet domestic needs and expectations appears to come up short—a problem Russia, Iran, and, increasingly, China have had.

Schmitt then goes on to make clear that this isn't just some abstract  exercise:

Of course, the question is: Why should we care? None of the three states directly threatens the United States. Indeed, arguably, if relations are tense, it is largely because Washington has pushed back against revisionist efforts—often about matters far from our shores and at times over issues for which we have no formal opinion (for example, about who has sovereignty over this or that islet in the South China Sea), no treaty obligation (as with Georgia or Ukraine), or no historical tie (as in Syria).

The answer is that, since World War II's end, Washington has understood that, strategically, Europe, East Asia, and the Middle East are the three most important theaters to the United States and that general peace and global prosperity depend on deterring non-liberal, would-be hegemons from disrupting those regions' stability. If history is any guide, the lesson learned has been that ignoring trouble on those fronts only postpones the difficulty and raises the cost of eventually dealing with it.

He takes a square look at the choices the US faced concerning the actions it has taken so far in each sphere:

Undoubtedly, the Great Recession of 2008 and the costly, indecisive wars in Afghanistan, Iraq, and Libya have soured large segments of the American public and their representatives on adopting a forward-leaning American strategic posture. With economic problems at home following the recession of 2008, the benefits of such efforts have appeared less than satisfactory.

But this invites two questions: What would the regional and geopolitical situations have been if Washington had not acted? And, as noted already, were the indecisive results a product of strategic overreach, flawed implementation, or a lack of sustained commitment to the task at hand—or some combination of these? The point is not that a forward-leaning posture can prevent costly policy mistakes but rather that one should not simply assume that the larger strategy is to blame for those mistakes. 

Nor should we assume that we cannot afford a forward-leaning strategy for Eurasia. Although its primacy is more contested today than in the after-math of the Cold War, the United States remains the world's only superpower. And while the West—the US and its democratic allies—has seen its overwhelming share of global economic and military power shrink in recent years, it still accounts for some 60 percent of the world's wealth and military spending. Moreover, although the contesting, revisionist powers have the advantage of operating in their own neighborhoods—meaning the US has the more complex and diverse task of responding to challenges far from home—the US has significant, close allies in each region that have begun to spend more on their militaries in the face of the threats posed by China, Iran, and Russia. 

Nor have the wars in Iraq and Afghanistan bankrupted the US. At the height of the campaigns, total defense spending (personnel, procurement, operations, etc.) as a percentage of GDP never rose above 5 percent, well below Cold War levels. Today, the base defense budget hovers at 3 percent or less.

In short, if the US and its allies wanted to do more to contest these revisionist powers in the realm of hard military power, they could. It is really a matter of policy choices and priorities. 

And here's a bracing truth that is hard to fully let in - but that must be let in nonetheless:

 China, Iran, and Russia have all read Washington's reluctance [to continue to lead as it had since the end of World War II] as an opportunity to advance their own plans and have done so in a manner that the American public has noticed. Even absent a major confrontation, American politicians may sense greater instability and greater prospects for conflict. This may lead them to argue the case for reversing course and, with the help of our allies, obtaining the benefits of deterring and containing the revisionist powers. To paraphrase Tocqueville, when it comes to American statecraft, Americans need to relearn the merits of acting on "self-interest rightly understood"—that is, looking not simply to one's immediate interest, but understanding that today's sacrifice may produce a longer-term and more substantial advantage.

We come back to that inescapable observation: None of these three powers, and indeed, none of the smaller and less powerful countries that reside in their spheres of influence, has anything like the values that inform the American character.

And those values have been diminished and distorted elsewhere in the West.

In short, America is exceptional and indispensable. We must lead.

There's that business about nature abhorring a vacuum.