The biggest threat to America’s security and prosperity comes not from abroad but from within. The United States has jeopardised its ability to act effectively in the world because of runaway domestic spending, underinvestment in human and physical capital, an avoidable financial crisis, an unnecessarily slow recovery, a war in Iraq that was flawed from the outset and a war in Afghanistan that became flawed as its purpose evolved, recurring fiscal deficits, and deep political divisions.

For the United States to continue to act successfully abroad, it must restore the domestic foundations of its power. Foreign policy needs to begin at home, now and for the foreseeable future. In other words: less foreign policy of the sort the United States has conducted since the Second World War and more emphasis on domestic investment and policy reform.

Since the publication of my book Foreign Policy Begins at Home last year (which Jacob Heilbrunn reviewed in American Review) I have been making these arguments across the United States. For someone like me, a card-carrying member of the foreign policy establishment for nearly four decades, my thesis borders on heresy.

What got me to this point? More than anything else, it started with the second Iraq war (begun in 2003) and the Afghan troop surge initiated in 2009. I mention both because my differences over the trajectory of American foreign policy are not with a single party. Many participants in the foreign policy debate in both parties appear to have forgotten the injunction of former President John Quincy Adams that America “goes not abroad in search of monsters to destroy,” along with the lessons of Vietnam about the limits of military force and the tendency of local realities to prevail over global abstractions.

As was the case with Vietnam, neither Iraq nor Afghanistan (as of 2009) was a war of necessity; more important, neither was a justifiable war of choice. In both cases, the interests at stake were decidedly less than vital. In both cases, alternative policies were available that promised outcomes of comparable benefit to the United States at far less cost. And in both cases, history and even a cursory study of the societies in question suggested that ambitious attempts to refashion the workings and political cultures of these countries would founder. What is more, all this was predictable at the time.

Now, with the advantage of hindsight, we can see that more than a decade of enormous sacrifice has hurt this country’s reputation for judgement and competence and failed to produce results in any way commensurate with the human, military, and economic costs of the undertaking. Such an imbalance between means and ends makes no strategic sense at the best of times. It is even less defensible now, when the United States faces difficult challenges to solvency.

To be sure, the United States is and will remain for some time first among unequals. It boasts the world’s largest economy and most capable armed forces. There is no peer competitor on the horizon. Nor has the United States acted in a way that has provoked a direct challenge. Add to this America’s unique demographics, the best higher education system in the world, and the potential for renewed economic growth, and it is clear that the 21st century could mark the second great American century.

There is nothing inevitable, however, about American sway over this young century. The advantages the United States enjoys are neither permanent nor sufficient to ensure continued primacy. The country is clearly underperforming. What makes the situation particularly worrisome are a large number of internal developments, including a burgeoning deficit and debt, crumbling infrastructure, second-class schools, an outdated immigration system, and the prospect for a prolonged period of low economic growth.

Many of the foundations of US power are eroding; the effect, however, is not limited to a deteriorating transportation system or jobs that go unfilled or overseas owing to a lack of qualified American workers. To the contrary, shortcomings at home directly threaten America’s ability to project power and exert influence overseas, to compete in the global marketplace, to generate the resources needed to promote the full range of US interests abroad, and to set a compelling example that will influence the thinking and behaviour of others. As a result, the ability of the United States to act and lead in the world is diminishing.

It helps to think of national security as a two-sided coin. One side is foreign policy – what a country does abroad, be it diplomatic, military, or in some other realm. The other side is more internal or domestic – all that a country does (or fails to do) to strengthen its economy and society. A country’s national security reflects what it is doing in both domains. Grand strategy is what a country does to advance its national security.

It is thus necessary to add a third threat to US national security, one related to but different than both over-reach abroad and underperformance at home. Call it “under-reach”: the risk posed by what appears to be a growing lack of understanding by many Americans of the close relationship between the state of the world – how much stability, how much prosperity – and the state of the United States. The result is that isolationism is making something of a comeback.

Like under-performance and over-reach, isolationism stems from within the body politic and crosses party lines. And as is the case with political dysfunction, it raises questions in the minds of others about American reliability, something that tends to lead friends to act more independently and foes more assertively.

Isolationism can be a deliberate strategy or the result of cavalier disregard for the implications of domestic policies for the US role in the world. It can be spurred, as well, by those who exaggerate the costs of what it takes to be effective in the world and the consequences of those costs for what needs doing at home, be it improving schools or infrastructure. The reality is that at or near current levels of spending on defense there is no guns versus butter tension in US national security; to the contrary, the United States can and should have its cake and eat it.

No matter what the inspiration, the emergence of modern isolationism is deeply troubling. The United States cannot thrive at home in a world of turmoil – and the world will move in the direction of turmoil absent consistent American leadership. This is not hubris but a statement of fact: order cannot be expected to just materialise, and no other country or group of countries has the capacity and commitment to bring it about. American foreign policy needs to begin at home, but it cannot end there.

Expressed differently, isolationism would be folly. Even if it wanted to, the United States could not wall itself off from global threats such as terrorism, nuclear proliferation, trade and investment protectionism, pandemic disease, climate change, or a loss of access to financial energy or mineral resources. The US government must be active in addressing these threats.

But the United States must also become significantly more discriminating in choosing what it does in the world and how it does it. Hard choices need to be made. It is not simply that it needs to recognise that the limits to its resources require it to be exacting in setting priorities; it must also recognise the limits to its influence. The United States needs to rethink what it seeks to accomplish abroad. Americans must distinguish between the desirable and the vital as well as between the feasible and the impossible. It also means Americans must resist wars of choice where the interests at stake are less than vital and where there are alternatives to the use of force. We cannot remake other societies in our image.

For the past two decades, American foreign policy, consumed with remaking large parts of the greater Middle East, has quite simply overreached. There is a strong case to be made that US attention and efforts should be better distributed around the world, with greater focus on the increasingly critical Asia-Pacific region and the Western Hemisphere and somewhat less on the Middle East. There is an even stronger case that US foreign policy should focus not so much on what other countries are within their borders and more on what they do outside their borders.

To mount an effective foreign policy, the United States must also put its house in order. That means fixing broken public schools, repairing or replacing aged infrastructure, putting into place a comprehensive energy strategy, modernising immigration policy, reforming health care, negotiating new trade accords, lowering individual and corporate taxes, reining in spending on entitlements, and reducing debt as a share of GDP. These steps would facilitate a return to the high levels of economic growth that America enjoyed in much of the post–World War II era but that have been out of reach over recent years.

It is not too late for the United States to put its house in order. It is not simply a case of necessity; currently it has an extraordinary opportunity to do so. The world is a relatively forgiving place now and for the foreseeable future. There is no 21st century equivalent to what Germany was in the first half of the 20th century and the Soviet Union was in the second.

The alleged other great powers — China, the European Union, Japan, and Russia — are not all that great. None has the means to overthrow the existing order and none is committed to doing so. Each is largely occupied with its own economic, social, and political problems. Meanwhile, challenges from the likes of Iran, North Korea, and al Qaeda, while significant, are neither global nor existential. The point here is that the United States is fortunate to have something of a strategic respite.

What stands in the way of the next American century, however, is American politics. To paraphrase Walter Kelly’s comic strip Pogo, we have met the problem, and we are it. Special interests often crowd out the general national interest. Partisanship can be healthy, but not when it leads to an inability to govern and to make difficult choices. This is especially the case, considering the 21st century’s 24/7 internet and media environment.

Either Americans resolve their political dysfunction, rethink their foreign policy and restore the foundations of American power — and in the process provide another century of American leadership — or the United States will increasingly find itself at the mercy of what happens beyond its borders and beyond its control. Such an outcome would not be in the interests of either the world or the country. The good news is that such a future can be headed off if the United States does what most Americans already know needs doing.