I was curious to find out just how often Congress has issued a formal declaration of war and the result can be found here:
An interesting passage is here:
The United States has formally declared war against foreign nations five separate times, each upon prior request by the President of the United States. Four of those five declarations came after hostilities had begun. James Madison reported that in the Federal Convention of 1787, the phrase "make war" was changed to "declare war" in order to leave to the Executive the power to repel sudden attacks but not to commence war without the explicit approval of Congress. Debate continues as to the legal extent of the President's authority in this regard.
And as far as approval goes, while formal declarations have been rarer, you also find plenty of instances of congressional authorizations that are below formal declarations. i.e. Iraq, the barbary wars were never formally
declared but they were authorized.
At the very least, looking at the historical record indicates that the idea that there must be a formal declaration of war for it to be legitimate (not talking about the wisdom of the wars, more the legality that Glenn hinted at) is false. More than that, it cannot be claimed that this was a recent perversion of the original intention, you had founders/presidents go either way depending on the occasion.
Another question on all this is if a formal declaration is not needed to wage war legally, why even bother with it in the first place? Personally, I think the standard for a formal declaration of war should be limited to more substantial TOTAL Wars, the kind you found in WWII, the stakes should be higher, but maybe you all can convince me that is foolish and insane. Deposition of leadership is a much more substantial goal than say, stopping pirating and kidnapping. Of course Iraq was a deposition of leadership among other things, so maybe the scales of the conflict factors in as well, and maybe optional wars don't qualify by default.