In an essay worth reading in full, James Fallows writes on America's idea of itself, and how that shapes our foreign policy:

The United States' external relations have often been volatile and have frequently seemed contradictory or even hypocritical, because of a tension built into the nation's identity from the start. The United States is both a nation and an idea. As a nation, it has the traditional interests of all other state players in power politics--advancing its businesses, defending its citizens and territory. As an idea, it is meant to appeal to people around the world regardless of nationality. Most Americans assure themselves that, in the long run, the two roles are more or less aligned, and that in expanding US power they will also expand liberties for human beings generally. But obviously there are contradictions and trade-offs. To give only the most obvious example: through the 1950s and 1960s the United States supported dictators in the larger cause of containing Soviet communism, and it makes similar trade-offs every day.

One way the American pubic handles these contradictions is through internal political criticism of the country's role in the world: congressman Abraham Lincoln opposing the Mexican-American War; Mark Twain opposing the Spanish-American War; Senator J. William Fulbright opposing the Vietnam War. But most citizens prefer not to think that their country is doing harm in the world. This is a part of self-protective human nature, which may be a larger than normal part of American nature.

So in the United States, the main result of the contradictions built into the nation's foreign policy is a ceaseless veering back and forth. For a while, the nation will act in a mainly self-interested way, until it feels too much like an ordinary, petty, self-interested state and not true enough to its identity as the first nation founded upon an idea. Then it will act mainly on principled grounds, or its definition of principle, until it feels that it is paying too steep a penalty in practical terms. The shifts are easy to tick off in overall policy: the United States was slow to enter World War I, on grounds of self-interest; but once in, under Woodrow Wilson it was ready to rebuild the world on the idea of democracy. As that proved frustrating, it withdrew in the 1920s to tending its own business again. It intervened deeply and for a decade in Vietnam; was so wounded in the process that it averted its eyes to the subsequent Khmer Rouge slaughter in Cambodia; and eventually was ready to intervene in Iraq again. Similar cyclical shifts also show up in policy towards specific nations and regions, notably China and the oil-producing states.

Ideally, the United States would avoid these vacillations with a policy that was always guided by morals yet not moralistic, that was always aware of practicalities yet not venal. The greatest leaders in their greatest moments managed such a balance, for a while. The rest of the time, leaders have been too practical, or too idealistic--and after a while, the electorate has grown uncomfortable and responded to leaders who promised to shift back the other way. After the disgraced Richard Nixon, the upright-sounding governor, Jimmy Carter. After President Carter, who seemed overwhelmed by complexities and care, the confident-sounding governor, Ronald Reagan.

What is unusual about the US's latest change is that the electorate was reacting against excesses in both directions. The Bush-Cheney administration managed to seem simultaneously too crassly blind to principle, as when setting aside Constitutional checks observed even during World War II, and too fancifully ideological, as when turning a blind eye to the difficulties and tragic pitfalls involved in trying to occupy and democratise a country it had just invaded.

That excerpt gives way to a discussion about Barack Obama's approach to grappling with these challenges. Read the whole thing.