So we are agreed that Congress can declare war-period.
The US has had a number of wars which might well be considered 'agressive' but that is for the American People to decide, hence the right of Congress (the part of government closest to the People), to do so and to face the judgment of the American People when they do so.
What is illegal is the Executive branch acting without any concern of what Congress says.
While the President of the United States has always had the Constitutional authority to act as the Commander-in-chief of the armed forces, conduct foreign affairs, repel invasions, and conduct certain other military affairs without prior approval of Congress, the right to declare war and thereby initiate a belligerency not already existing was reserved to Congress. The exact circumstances in which these facts operate remain highly controversial and debatable. The circumstance in which the Congress may elect to authorize a war of aggression in violation of the current laws of armed conflict is beyond debate and flatly illegal.
So we are agreed that Congress can declare war-period.
No, on the contrary, are not agreed at all. The Constitution said in 1787 that the Congress has the power to declare war, while the the power to make war was reserved only to the President. The Congress subsequently made binding treaties which made it unlawful under international law for the Congress to utilize its Constitutional power to commence a war by declaring war. A similar situation exists with respect to the Constitution giving the Congress the power to grant letters of marque and reprisal to privateers. The international law changed and made letters of marque and reprisal illegal under international maritime laws, and the Congress complied with international law by abandoning its Constitutional power and making letters of marque and reprisal illegal under Federal law. The Kellogg-Briand Pact and the United Nations Charter forbid the commencement of war, and the Constitutional power to declare war is the power to commence war. Consequently, the prevailing understanding at the moment is that international law forbids Congress to utilize its Constitutional power to commence a war with a declaration of war. The Founding Fathers were very particular in observing that they and the U.S. Constitution were obligated to respect international law.
What the Kellogg-Briand Pact and the United Nations Charter did not forbid was the right to make war in self-defense against a sovereign who commenced a war. On the contrary, those authorities require members to restore peace by negotiation and by making war in self-defense. Consequently, post Second World War hostilities have been conducted under the authority to make war in self-defense of the United States and/or the self-defense of member states against whom an aggressor/s have first commenced a war. The Constitution does not state the Congress must exercise its power by a formal declaration of war. Instead, the Constitution only grants the power to declare war, without defining the method/s by which the war is to be declared. Congressional resolutions authorizing the use of hostile military force has since been recognized as the means by which Congress exercises the power to dclare war in self-defense without a formal declaration of war that could be misrepresented as the older form by which a war was commenced by other sovereigns for the purpose of aggression.
NATO conducted military operations against the Government of Libya under the legal theory that ithe Libyan Government's war upon unarmed civilians invoked the U.N. Charter and its authority for members to conduct peacemaking operations in self-defense of the world community and the civilians.
Since the Constitution gave the power to make war and conduct foreign affairs to the President and the U.S. Congress had already authorized the President to comply with the NATO and United Nations treaties, the Presidential Administrations have typically interpreted the Constitution as having given the Commander-in-Chief the power to make war in compliance with already existing authorities to make war in a coalition for self-defense.
The Constitutional question as to whether or not the Congress had the power to limit the Constitutional power of the President and co-equal Executive Branch of government with the War Powers Act has never been ajudicated and settled.
The US has had a number of wars which might well be considered 'aggressive' but that is for the American People to decide, hence the right of Congress (the part of government closest to the People), to do so and to face the judgment of the American People when they do so.
The US is unlikely to have engaged in any international wars which would qualify under international law as being aggressive war making. Even the military operation against Libya was very clearly a military operation implementing the United Nations Charter mandate to conduct peacemaking operations in response to crimes against humanity.
What is illegal is the Executive branch acting without any concern of what Congress says.
That statement misrepresents the situation. The fact of the matter is that the Congress may have tried to close the barn door long after the horse ran out the barn door. In other words, the co-equal branch of government may be on good legal grounds to assert the Congress already authorized the United States and its Commander-in-Chief to make a war of self-defense under the NATO and United Nations authority to conduct peacemaking operations. This means the Congress may already have exercised the power to declare war by authorizing the United States to make war in the NATO and United Nations grants of authority, so Congress would have limited powers to retract the powers already exercised.