What can the United States offer its allies? Throughout the Cold War, the answer was simple: the United States guaranteed its allies security from the Soviet Union. But this question - which seems so basic - is difficult to answer today.

It is undoubtedly true that the United States remains the predominant military power on Earth - and that countries as diverse as Canada, Saudi Arabia, Turkey and Japan (along with many others) depend on American military power to provide security for international commerce as well as for baseline, worst-case scenario security guarantees. There is also little doubt that the United States obtains strategic benefits in exchange for these services in the form of energy supplies, cooperation against common threats, and more.

The problem is that this power - while immense - is not very fungible. That is, the United States cannot easily threaten to withhold a portion of its security guarantee or its protection of international waterways if (say) Turkey chooses not to support the United States' policy toward (say) Iran. Compounding the problem is that the worst-case scenarios in which American military power would be necessary are more difficult to imagine today.

This is an increasingly important problem as the United States tries to reorient its strategic objectives and relationships to address today's challenges.

Let me explain by way of the Turkish example.

During the Cold War, the United States and Turkey formed a "strategic partnership" based on both countries' fear of Soviet intervention in the Middle East. The Truman Doctrine offered a specific guarantee that both Turkey and Greece would be protected from Soviet aggression - a fear that was quite real in Turkey at the time. In exchange, the United States received access to military bases, support in the Korean War and a strategically advantageous position in the Middle East. Despite serious disagreements - particularly over Cyprus - the relationship worked to each sides' mutual advantage until the Berlin Wall fell 20 years ago.

Today, the United States wants Turkish support on a wide variety of important issues, including stabilizing Iraq, supporting the mission in Afghanistan, preventing Iran from obtaining a nuclear weapon, moving energy to Europe, serving as a Muslim ally, and providing stability in its neighborhood.

In exchange, the United States offers security guarantees, military assistance, and the benefits that accrue from an alliance with the world' most powerful military. All of these things are very important to Turkey (and to many other countries). The problem is that the United States is not in a position to credibly threaten to withhold these benefits without undermining the international order in which it has invested so much. For example, both Washington and Ankara know that Turkey's stance on Iran's nuclear program will not jeopardize the American security blanket.

Of course, there are red lines that Turkey (or any other country) could cross that would change U.S. policy. But the point is that Turkey has a great deal of running room before those red lines are crossed. Turkey, both because it is a NATO ally and a strategically critical country, knows that it can pursue an independent foreign policy while still enjoying the benefits of American power.

From Tokyo to Paris - and many places in between - it is not so much the lack of American power that is the problem (it still has plenty), but rather the fact that its bargaining position is paradoxically undermined by its extraordinary role.

-- Ben Katcher