Is a declaration of war necessary for the country legally to be at war?

(Updates added at end of posting)
Well, no. Eugene Volokh discusses the issue with a lawyer's mind. A reader wrote him to say that because there had been no declaration of war by Congress, the country is not actually at war. Eugene responded:

International law, domestic law, and common linguistic usage have all long recognized that a nation can be at war without a declaration of war. The law and usage both focus on what is in fact happening, and not just on whether certain magic words have been used. The U.S. was at war during the Civil War, during the Korean War, and during the Vietnam War, though to my knowledge there were no declarations of war then. There is simply no legal or linguistic rule that says the contrary.

This is an important point. Most people are surprised to learn that Congress never declared war during the bloodiest conflict in American history, the Civil War. (In fact, many southern senators and congressmen retained their seats and activity in the Congress.) The Lincoln administration's position was that the Union was suppressing a rebellion, not actually making war in the then-accepted legal sense of the word. (Ask an Antietam survivor whether he fought in a war, though.)

Because of the federal government's position, foreign powers kept their hands out of what they perforce treated as an internal American matter. But when Lincoln ordered the Anaconda Plan to be implemented by the Union Navy, he almost lost that international forebearance. Under the plan, the Union Navy blockaded southern ports. But under international customs and usage of the time, blockades were legally understood as acts of war by one sovereign power upon another - an equal, in other words. Great Britain almost recognized the Confederacy as a result of the blockade, because of the implication that the US federal government was treating the South as a de facto sovereign state. It took a major diplomatic effort by the US state department to prevent England from recognizing the South.

Also, remember that the Constitution empowers the Congress to make war, not just to declare it. But the Constitution does not require the Congress to declare war in order to make war. Warmaking is nothing other than the employment of US military forces to achieve US national objectives, involving actual combat (re. Clausewitz: "Without killing there is no war").

Employment of forces to achieve national goals without combat is within the Constitutional authority of the president as Commander in Chief. President T. R. Roosevelt sent the Navy's Great White Fleet on a world cruise. Congress opposed the cruise and refused to fund it, but Roosevelt sent the fleet anyway. There was no combat so Congress couldn't do anything about it. Roosevelt was Commander in Chief and could send the fleet anywhere he wanted to. In 1988, the communist Sandinista army of Nicaragua mounted a major incursion into Honduras. President Reagan sent a brigade of the US 82d Airborne Division plus some support units to Honduras as a national demonstration. The Democrats howled but couldn't stop it. The Sandinistas beat a fast retreat and after several days the 82d came home.

Update, late June 5
Eugene Volokh graciously wrote me, "I'm glad we generally agree, but I'm not sure whether the Constitution in fact empowers Congress to make war. My sense is that it empowers the President to make war (via his commander-in-chief power), but unless I'm missing something -- and I well might be -- I'm not sure it actually gives Congress the power to make war. Am I mistaken?"

I emailed him back, but since his expertise is certainly superior in Constitutional matters than mine, I'll await his reply before posting. He is correct (of course) that the Constitution does not specifically empower the Congress to "make" war, at least not in those words.

Update, early June 6
An anonymous correspondent writes,

Under the National Security Act, there are certain military response procedures, which are pre-approved. Neither the Executive nor the Legislative branches control day to day aspects of military operations. That is: certain legal aspects of military options are already settled. NSA protocols form effective declarations of war, which are pre-fabricated by lawmakers.

This is a good point; the president doesn't have to run over to Capitol Hill for permission to act if there is an attack on the United States. Congress has, through legislation, approved certain military responses to attack. But there has never been any blanket authority granted to the executive to conduct war. I would respectfully disagree with the correspondent that the legislation forms "effective declarations of war," because they do not grant the president authority to conduct operations other than responses. No offensive campaigns are authorized, for example, certainly no extended ones. However, presidents would probably take a more permissive view of the legislation than would Congress:
Congress: We didn't authorize that!
President: Yes, you did, it says so right here!