The thing about the law or the rule of law, or written law, or laws in general, is that in America, politicians by a wide margin are lawyers, and lawyers have a strong tendency to interpret law in whatever manner favors the policy that they wish to enact or to enforce. Our Constitution makes it clear that: "the congress shall have the power to…. Declare war …. Raise and support armies…." and so forth, so that it is very unambiguous that in the three branches of our Constitutional government, that it is the Congress, which as the legislative branch, that is given the power to declare war, and not the Executive branch, which is the Presidency.
The two biggest wars ever fought by the USA were wars declared by the Congress, for World War I, and for World War II, to which because these wars were voted on and passed by Congress, means, that the people through their representatives in Congress had their say. However, since, World War II, America has been at war on numerous occasions to which an expressed declaration of war, was not formally declared by the Congress. In some of these cases, such as the War in Afghanistan, authorization by Congress was given to the Commander-in-Chief, that is, the President, to engage in military combat with a foreign nation, but without the formal declaration of a war, even though, in effect, all the actions of America in these types of military affairs, were in accord to what most reasonable people, would see as being an actual war. But, whether considered to be a war or a military engagement, at least there was a discussion and a vote by Congress, before military intervention and action was taken.
There are, though, on the other hand, numerous instances of the President, on his own, without Congressional approval, going to war, accomplished for example, against Libya, Kosovo, and Syria, all over the last twenty-odd years. However, by far the biggest undeclared war by America, costing the lives of approximately 54,000 soldiers of America, was the United States war against Korea, as authorized by President Truman, to which, as for his reason for this war, Truman stated that it was on behalf of the United Nations resolution passed by its Security Council.
If America, wanted an Imperial Presidency, who on their own, without authorization by anyone else, to be able to declare war or the actions consistent to what war is about, than the Constitution would have clearly and unequivocally supported such an action. In fact, the Constitution does not support that action, and while there is blame to be placed on Congress, itself, for not asserting itself collectively against this breach of faith and power by the Presidency, it is the President, himself, that has usurped this power.
Unfortunately, for the citizens of this mighty nation, the fact that the President, can declare war at any given moment, means that the United States will be and is in a perpetual state of readiness for war, with its attendant massive amounts of materiel, and personnel to carry out these orders swiftly and effectively, ever ready for wars that will protect our foreign entanglements, as well as to support and protect our mega-international corporations, and to protect our access to whatever natural resources America feels it has a God-given right to, and most importantly so that the military-industrial complex that runs this nation, only has one person, the President, that they need to manipulate so as to enable these purveyors of war, to profit and to make their blood money so as to satisfy their lust for power and filthy lucre