In earlier times, under the laws and customs of armed conflict, a sovereign was the only party who could lawfully decide when to engage in a state of war against another sovereign. Among the Republican Romans and Imperial Romans, there was a ceremony involved with the decision which entailed notice being given to the enemy by the tossing of a spear into enemy territory. Roman officers who engaged in war against an enemy with whom a truce existed or without the authority of the Roman Senate was surrendered to the enemy for justice and punishment upon complaint from the enemy.
Conversely, the Roman Senate refused to punish a captive Roman officer who made treaty with enemy pirates for ransom in exchange for liberty and then abrogated the treaty under the theory any person could lawfully make war upon and punish pirates and bandits outside the protection of the law, i.e. outlaws.
The lawful authority for a sovereign to engage in a state of war against another sovereign continued into the early 20th Century. A declaration of war was the formal document used to acknowledge the existence of a state of war, especially in those instances where acts of war were committed and remained ongoing even while the formal declaration of war acknowledging the informal state of affairs was under debated, finalized, and presented to the belligerent sovereign. The cessation of diplomatic relations establishes a state of belligerency, while the declaration of war acknowledges the hostilities underway constitute a formal state of war under which certain other formal relationships may change between the belligerents and neutral sovereigns.
Kaiser Wilhelm of Imperial Germany utilized his authority as sovereign of Germany to plan a number of wars for Imperial conquest. Several plans he ordered to be prepared included the German invasion of limited regions of the United States as revenge for interfering with German Imperial military operations elsewhere in the world. When these plans failed to be implemented for a variety of reasons, he undertook plans to start a war in Europe, which resulted in the First World War. Afterward, many authorities clamored to bring Kaiser Wilhelm to justice for his role in causing and engaging in the war. When the efforts to prosecute the Kaiser failed, his accusers set out to use the new League of Nations as the means of outlawing the right of a sovereign to engage in a war of conquest. The only right to engage in war would be the right to fight in war to defend against an offensive act of war. This goal was achieved with a new treaty.
The Kellogg-Briand Pact for the first time in history denied the signatory sovereigns the right as a sovereign to unilaterally breach the peace and engage in aggressive war. Some sovereigns, however, defied their treaty obligations by falsely claiming their acts of war were the permitted defensive acts of war. In response to the breaches of the Pact and attempted circumventions, the Stimson Doctrine of the League of Nations and the Budapest Articles of Interpretation adopted in 1934 made it a requirement for sovereigns to employ nonrecognition of the aggressions and the right and obligation of member sovereigns to preserve their own neutrality while aiding sovereigns engaged in defensive war against an sovereign/s engaged in aggressive war.
The aggressors soon withdrew from the League of Nations and repudiated their treaty obligations mandating a refrain from aggressive war. The aggressions continued until the sovereigns engaged in collective defensive war found it necessary to issue declarations of war in response to the ongoing hostilities by the aggressors. The Second World War brought about the dissolution of the League of Nations, which lacked the powers needed to engage thee defensive war required to deter and defeat sovereigns engaging in aggressive war. In the place of the league of Nations, the United Nations engaged in their defensive war against the aggressors established their new organization. This organization was initially founded with the authority for the members to engage in defensive peacekeeping operations to deter acts of war and defensive war to force a halt to wars of aggression.
Since the Kellogg-Briand treaty remained in effect to outlaw wars of aggression, the United Nations established a Security Council to enforce the peace and use defensive war to enforce its decisions. The Charter outlawed war except in self-defense and collective self-defense against an aggressor. It is this development in international law of armed conflict which brought into question the legality or illegality of a declaration of war.
In the past, a declaration of war was used by the aggressor and/or the defender to formalize a state of war. After the Kelloggg-Briand Pact and the pacts and treaties leading to the UN and the Security Council outlawed aggressive war, the defender could act in self-defense with or without the approval of the Security Council, while the aggressor could not use a declaration of war to engage in an illegal aggressive war. This situation left the declaration of war in a state of limbo. Since the defender did not need a declaration of war to authorize its state of war, and the sovereign issuing a state of war could in some circumstances appear to be declaring an aggressive state of war, rather than a defensive state of war. Consequently, the declaration of war fell into disuse to avoid the appearance of being the aggressor in violation of international law.
The U.S. Constitution, having been adopted in 1787, contemplated the use of a declaration of war when it was deemed necessary for the Congress to formally initiate a state of war against an enemy belligerent or formally recognize a state of war against an enemy already engaged in hostile aggressions. 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. This is why you often hear it said that a declaration of war is obsolete and illegal. While it may not be altogether illegal, there is a general perception that lawful defensive wars require no declaration of war, while a declaration of war may be regarded rightly or wrongly as an illegal exercise of aggressive war. The right of sovereigns to engage in collective self-defense against an aggressor engaged in acts of war against any one or more members of the collective sovereigns is guaranteed by the Kellogg-Briand Pact and all subsequent treaties up to the United Nations Security Council. NATO is one of those sovereign collective defense organizations intended by the international laws, and the United States acted against Libya as a member of NATO. Since the Libyan operations did not require the President to repel and invasion or similar act contemplated by the Founding Fathers, it is unclear the extent to which the NATO treaty obligations approved by Congress may be regarded as pre-authorization for hostile acts against Libya without further approval of Congress beyond the authority of Congress' war Powers Act. This is an area which remains in controversy, so the Constitutional legality of using a declaration of war when waging defensive war unilaterally or collectively is in question.
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.