As Iraq unravels, a painful truth about politics and foreign policy appears more evident: The United States is very good in all-or-nothing situations, but all-or-nothing situations don't often arise.
This is a country that can and will meet existential threats with unity of purpose and vast resources. In this regard, even now, it stands alone. Few threats rise to that level. Lesser dangers can still be serious, without commanding or justifying that kind of response. Precisely for that reason, they put greater stress on democratic politics, and U.S. politics seems ever less able to cope.
The "war on terror" is the paradigm example of this syndrome. In Iraq, Syria and Afghanistan, nothing is simple. The goals are complex, the trade-offs excruciating. On this subject, Anthony Cordesman's papers on Iraq and Syria are particularly worth studying. They're full of warnings about the new security challenge. These now look prescient — and they highlight the peculiar difficulties that confront the indispensable nation. His most important lessons are especially hard for the U.S. to follow.