The current war on terror often raises the issue of why war has not officially been declared against the enemy. Most people don't realize that the United States has not declared war since World War II (when a number of countries, not just Japan and Germany, were so named.) And there's a reason for that, one that is rarely discussed.
Seems that after World War II, Congress wrote into law a lot of the wartime measures used during World War II. These included price controls, censorship and greater police powers. This was done with the possibility of nuclear war in mind, where there would be massive damage done to the U.S. in a short period of time. To deal with this, a lot of these regulations would kick in the minute Congress votes to declare war. No one wants to be the first to suggest repealing these laws and regulations, and no one wants to see them go into action. So whenever anyone in Congress starts talking about declaring war, they are pulled aside by some senior staffers and filled in on the consequences.
Of course, the war on terror is more complicated because there is no country to declare war on. That, of course, could be easily gotten around, by crafting a declaration that names an institution (al Qaeda, et al). But it's so much easier just to pass a resolution of support for the president, and let the chief take a beating if it all goes to hell. Congress can always say, "we didn't declare war," without getting into the real reason why not.