5. Congress Has Checked Out.
The authority to declare war is given to Congress, not the president, but that authority has been steadily usurped ever since World War II. Although the Constitution could not be clearer on this point, modern presidents clearly feel no constraints about ordering U.S. forces to attack other countries, or even to fully inform Congress as to what we might be doing in secret. In practice, therefore, the vaunted system of "checks and balances" supposedly enshrined in our Constitution simply doesn't operate anymore, which means that the use of America's military power has been left solely to the presidents and a handful of ambitious advisors (see No. 4 above). This is not to say that public opinion doesn't figure into their calculations (i.e., they've got pollsters and political advisors too), but it is hardly a binding constraint.
I've no doubt that one could add more items to this list (e.g., the passive press, the military-industrial complex, etc.), but the items already noted go a long way to explaining why the supposedly peace-loving United States keeps finding itself in all these small but draining wars.
Back in the 2008 campaign, Barack Obama said that his favorite movie was The Godfather. And if I recall correctly, he said his second favorite movie was The Godfather, Part II. But his presidency is starting to play out like Part III of that famed trilogy, where Michael Corleone rails against the fates that have foiled his attempt to make the Corleone family legit.
I can just hear Obama saying it: "Just when I thought I was out … they pull me back in." Precisely.