Why do we no longer have a Declaration of War?

I have linked an interesting article from Stratfor that details how the process of America entering a war has changed since WWII, which is the last time an American President made a formal declaration of war. I do feel that I need to disclose that I'm not taking a political position by posting this article.  Reading the article in its entirety is recommended as it finds Presidents of both political parties at fault for removing the declaration as a procedure that was necessary to begin a military action.  Some interesting excerpts include:

A declaration of war, I am arguing, is an essential aspect of war fighting particularly for the republic when engaged in frequent wars. It achieves a number of things. First, it holds both Congress and the president equally responsible for the decision, and does so unambiguously. Second, it affirms to the people that their lives have now changed and that they will be bearing burdens. Third, it gives the president the political and moral authority he needs to wage war on their behalf and forces everyone to share in the moral responsibility of war. And finally, by submitting it to a political process, many wars might be avoided. When we look at some of our wars after World War II it is not clear they had to be fought in the national interest, nor is it clear that the presidents would not have been better remembered if they had been restrained. A declaration of war both frees and restrains the president, as it was meant to do.

I am not open to the possibility that quickly after the commencement of hostilities the president need not receive authority to wage war from Congress. And I am arguing that neither the Congress nor the president has the authority to substitute resolutions for declarations of war. Nor should either want to. Politically, this has too often led to disaster for presidents. Morally, committing the lives of citizens to waging war requires meticulous attention to the law and proprieties.
Read more: What Happened to the American Declaration of War? | STRATFOR