And perhaps the real answer to that critical last question might be this: That the largest effect of 9/11 upon America is that it became distracted. Distracted in two very important ways. In the first place, it was distracted from many other things that are going on in the world. Secondly, it's been distracted from the erosion of its financial strength and international competitiveness.
Let us look briefly at the first matter. In its own hemisphere - surely among the most important areas in the world to U.S. interests - a new Latin America is emerging, unsteadily but observably. There is human catastrophe in Haiti, an uncertain future for Cuba, the continued idiocies of a sick Chávez regime in Venezuela, and drug-gangster wars from Bolivia to Mexico.
Yet there is also the extraordinary transformation of Brazil, the success of Chile, and the quiet recovery of Argentina. But does the United States have a positive, carefully crafted strategy for Latin America? Of course not.
Africa, apart from a few lights of promise, trembles over the pit of environmental and demographic disaster, but Washington leaves that problem to the World Bank. Europe fades further away. Russia is neglected. An India-Pakistan policy is, well, hard to describe. And American views on China range from blind enthusiasm to calls to build up the U.S. Navy immediately. And all this neglect for adventures in Afghanistan and Iraq that are now being wound down. This will be hard to explain to history students in 50 years' time.
Even more worrying has been a decade of distraction from attending the "common wealth" - that is, the common good of America itself and of its citizenry.
The Bush administration's combination of expensive foreign wars and inexcusable tax cuts that favored the rich has had dreadful effects upon U.S. federal deficits, upon America's growing dependency upon foreigners, and upon the long-term future of the dollar.
The social fabric is fraying, the underclass is growing - it is observable year upon year in the soup kitchen where I work - and the public-school system crumbling. Under-investment in our roads and railways and power systems is visible every day. And, as if any more bad news was needed, along comes a Tea Party with policies that would make America's double-distraction even worse.
This, then, may be the real legacy of 9/11, long after U.S. troops are withdrawn from those high Hindu Kush ranges.
For here was the decade when America turned its attention away from its own domestic condition and from its need to have a wider view of global change.
David Fuller's view The two wars in response to 9/11 have certainly been very costly for the USA in both treasure and human lives. However, the trends in ballooning debt for US consumers and their government had commenced earlier, as had the worldwide conversion to capitalism which has made today's growth economies so competitive.
The necessary deleveraging by consumers is a lengthy process and the US government has yet to grasp this nettle. Nevertheless, there is no reason why these problems cannot be resolved, although perhaps not in the current political climate.
Many US multinational companies have seldom looked healthier, with the help of globalisation and despite the US economy's travails. A little over a decade from now the US should be self-sufficient in energy, mainly due to the development of its vast reserves of shale gas and shale oil.
By then, chronic personal indebtedness should be a distant memory for most Americans. Lessons learned become a platform for sound economic governance. For investors, the secular trend of valuation contraction in stock markets, which we have talked about since 2000, should be followed by the next lengthy boom.