American national-defense priorities and practices are being shaped by two important developments, peculiar to the modern age and in some ways defining elements of the contemporary period: the need for rapid American response to security threats around the globe and the inability or unwillingness of traditional United States allies to support American policies abroad.

Neither of these elements was prominent as the last millennium drew to a close; the farewell trumpets were accompanied by paeans to American power and American influence.

You don’t hear much of that anymore. The terrorist attacks of 2001 underlined the nation’s vulnerability in an age when the dispersal of technology threatens security rather than enhances it. The war in Iraq this spring underlined the nation’s isolation in the new world order.

That isolation may be a function of the mission (taking out Saddam Hussein) and the man (George W. Bush) who led the effort. In another circumstance, with another president, the world might well rally to America’s side.

But the fact that world leaders did not fall in line behind President Bush when he argued that Iraq was producing weapons of mass destruction makes it harder to believe that they might do so the next time, when the rationale might be less alarming. And, of course, the failure to discover chemical, biological or nuclear weapons in Iraq and the troubling intelligence failures, deceptions and errors in the Iraq episode will make it harder than ever for Bush or a future president to win worldwide support for any American initiative.

In short, by choice and by circumstance, the United States may stand alone.

First, the choice: The United States, for example, would like other nations to help rebuild Iraq, and to pay for that effort, but in exchange for the help and the money, the Bush administration is not willing to relinquish control of the operation there, which is making it hard for Germany and France to make substantial commitments and contributions.

Now, the circumstance: No nation has the wealth and power that America possesses, and as a consequence no nation is so rich a target — or so indispensable a player in world affairs. “At this moment in time it is American power, and American power only, that can serve as an organizing principle for the worldwide expansion of a liberal civil society,” diplomatic analyst Robert D. Kaplan writes in the current issue of The Atlantic Monthly.

And finally, the consequences: The Bush administration is girding to assemble a fighting force that is more mobile, lethal and capable of acting without the allied staging grounds that it failed to acquire in the run-up to the Iraq war.

The administration, to be sure, is building what defense analysts Thomas Donnelly and Vance Serchuk call “a worldwide network of frontier forts,” with outposts in places the United States seldom emphasized before, including Africa. Some of these posts, too, are in former republics of the Soviet Union, another unanticipated consequence of the end of the Cold War and the beginning of the age of terror.

But the United States is not betting on forts and frontiers alone. Just this month, for example, Secretary of State Donald H. Rumsfeld set in motion a dramatic transformation of the military that would shift responsibilities from reserves to active-duty personnel and permit American forces to mobilize for war in about a fortnight’s time. The policy memo setting forth this transformation, first unearthed by the Los Angeles Times, has an obvious thesis: The staging areas of the future might well be on American soil, not abroad.

The United States has been moving in this direction for some time, though more for convenience than for strategy. Many of the bombing missions in the U.S. military efforts in Central Europe, Afghanistan and Iraq began on U.S. bases, with American aviators returning to home soil, and their own beds, after each sortie.

But what the administration now has in mind is more sweeping — and more strategic. That’s why little-noticed missile-technology tests conducted in a Virginia wind tunnel this spring may be so important.

The defense experts refer to these tests as the Hypersonic Flight Demonstration Program (HyFly, to the rest of us), and the compelling quality of this technology is, as the Johns Hopkins University Applied Physics Laboratory people put it, its potential to “develop a future high-speed strike weapon to engage and defeat time-critical, heavily defended, hardened or buried targets while keeping forces farther from harm.”

Now that’s a mouthful, but the important three words are comprehensible to those of us who are not, as fate and SAT scores would have it, rocket scientists. The words are “farther from harm.” We should read those three words to mean that American forces eventually might not be, in a phrase we can borrow from World War I (and from George M. Cohan), “Over There.”

If this HyFly technology is extended successfully, increasing numbers of American forces would be Over Here, where, increasingly, American national-security concerns rest. The science involved in these weapons is remarkable; the missiles would travel six times the speed of sound and, for those of you keeping score at home, would only have to carry fuel, not oxidizer. (Today’s rocket engines carry both. Lots of defense-geek Web sites can tell you why.)

But the technology isn’t only what intrigues the strategists. With these missiles, the United States’ need for allies could be diminished ever further. The shorthand for this technology might be HyFly, but the byword is Go It Alone. It may be the missile of the future, but it also may be the mission of the future.

David Shribman is a columnist for the Pittsburgh Post-Gazette.