Bad as Obama policies are for America, they are equally dangerous for friends who have relied for decades on the U.S. nuclear umbrella as a foundation of their own national security strategies. As Washington’s capabilities decline and as it narrows the circumstances when it will use nuclear weapons, allies are asking hard questions about whether the U.S. nuclear umbrella will continue to provide the protection it has previously.

Many allies see clearly that our mutual global adversaries have no intention of reducing their own nuclear programs in imitation of Mr. Obama. Our friends accordingly feel increasingly insecure. If Washington will not continue to hold the nuclear umbrella that has provided strategic stability for so long, other countries will begin making divergent decisions about how to protect themselves, including, for some, the possibility of seeking their own nuclear weapons.

Within the administration, there are strong advocates for America pledging “no first use” of nuclear weapons. Although the nuclear posture review “only” expanded “negative security assurances” somewhat, there is little doubt that “no first use” is alive and well in internal administration councils. These self-imposed constraints on the use of nuclear weapons reinforce the allies’ concern that Mr. Obama has forgotten the central Cold War lesson about the U.S. nuclear deterrent. There was never any doubt that a Soviet attack through the Fulda Gap into Western Europe would have swept through NATO forces, possibly all the way to the English Channel. Thus, the threat of U.S. nuclear retaliation against such an attack – an unambiguous case of a U.S. first use of nuclear weapons – was precisely what was needed to keep Soviet forces on their side of the Iron Curtain.

The risks come not only from the Obama administration’s nuclear policies. By canceling the Polish and Czech missile defense sites, the president signaled that he has less than full faith in the concept of a U.S. national missile defense capability. Moreover, and equally important, Russia and others quickly interpreted the decision not to construct the Eastern European facilities as Washington backing down in response to Russian threats. At a minimum, Mr. Obama showed that he was prepared to use U.S. missile defense as a bargaining chip, exactly the misguided policy option President Reagan consistently and emphatically rejected. If America’s homeland remains vulnerable, its willingness to risk confrontation with an opponent will be substantially reduced. In such circumstances, U.S. allies could not count on the threat of nuclear retaliation by Washington in the event of aggression, as they could in the Cold War.

Accordingly, Europeans should be very worried that they are increasingly on their own to face the re-emerging threat of Russian belligerence. Because the New START treaty does not limit tactical nuclear weapons, Europe, simply because of geographic proximity, is most vulnerable to Russia’s advantage in that category. It is thus highly ironic that some NATO countries have recently called for removing the last U.S. tactical nuclear weapons from Europe, which will simply enhance Russia’s existing lead. Moreover, because the conflict in Afghanistan has opened new fissures in NATO, Europe must ponder whether the aging alliance can renew its original focus on defending against Moscow.

In the Pacific, concerns are equally acute, especially in Japan. Faced with the unambiguous reality of China expanding and modernizing its nuclear and conventional military capabilities, and with North Korea as a nuclear weapons state, Japan inevitably faces the question of whether it needs its own nuclear deterrent. U.S. ambivalence on missile defense only heightens Tokyo’s concerns, given its proximity to ballistic missile threats from the East Asian mainland. South Korea, Taiwan and Australia, among others, also share Japan’s concern, each according to its own circumstances.

Thus, while there unquestionably are variations among America’s allies about the precise implications of Mr. Obama’s global withdrawal from U.S. strategic nuclear dominance, the overall direction is not in doubt. U.S. decline leaves the allies feeling increasingly on their own, uncertain about Washington’s commitment and steadfastness and facing difficult decisions about how to guarantee their own security. Ironically, therefore, it is America’s friends that might increase nuclear proliferation, not just their mortal foes. This is the reality created by the retreat of nuclear America, the exact opposite of the Obama administration’s benign optimism, namely that reducing U.S. capability would encourage others to do the same.

John R. Bolton, a former ambassador to the U.N., is a senior fellow at the American Enterprise Institute and author of “Surrender Is Not an Option: Defending America at the United Nations and Abroad” (Simon & Schuster, 2007).