That's the line on the cover of a recent Economist and the title of its leading article. A supposed news article asserts "The decline of deterrence" because "America is no longer as alarming to its foes or reassuring to its friends." Both articles demonstrate a lack of understanding of either history or deterrence theory.
Start with what Americans have already fought for, among other things: independence for Spanish-controlled Cuba; freedom and independence for a lot of European countries; the status quo ante bellum in Korea and Kuwait; preservation of the comparatively democratic government of South Vietnam; overthrow of pretty nasty governments in Afghanistan and Iraq. Despite some rhetoric at the time, few of these places were direct vital interests of the United States.
Many people on both sides of the Atlantic during the cold war doubted that the United States would really risk the nuclear annihilation of New York and Washington in order to protect Bonn or Ankara. Whatever the reasons, deterrence worked then, and remains a strong possibility even today for Talinn and Warsaw. Nuclear weapons should -- and do -- make national leaders cautious. [They also provide a powerful incentive for proliferation: remember the lesson of the first Gulf War, as expressed by the Indian defense minister at the time: "Never fight the United States without nuclear weapons."]
The logic of forward deployment goes back a long way. In the secret Anglo-French military talks in 1910, the British general asked how many troops he would have to provide to defend France against a German attack. General Foch memorably replied, "Give me a single British soldier, and I'll see that he is killed on the first day." That's the logic behind even the small deployments of American military personnel to the Baltic nations and other allies -- not to prevent overwhelming attacking forces from succeeding at least at first, but to raise doubts about ultimate outcomes, and therefore to increase deterrence.
But since deterrence is psychological, the doubters risk creating a self-fulfilling prophecy.