With regard to the US and Iraq?
There are several reasons, but the separation of powers bit is enough in this case.
Declaring and Waging War: The U.S. Constitution
by Jacob G. Hornberger, April 2002
Excuse me for asking an indelicate question in the midst of war, but where does President Bush derive the power to send the United States into war against another nation? The question becomes increasingly important given that the president has indicated that once the Afghan War has been brought to a conclusion, he intends to use U.S. military forces to attack other sovereign nations.
It is important to keep in mind that our system of government was designed to be unlike any other in history. First, the federal government was brought into existence by the people through our Constitution. Second, the Constitution is the supreme law of the land that controls the actions of our public officials in all three branches of the federal government. Third, the powers of the federal government and its officials are not general but instead are limited to those enumerated in the Constitution.
Fourth, the government is divided into three branches, each with its own enumerated powers, and one branch cannot exercise the powers of another branch. Fifth, the Constitution expressly constrains democratic, majority rule. Sixth, public officials are not legally permitted to ignore any constitutional constraint on their power but must instead seek a constitutional amendment from the people to eliminate the constraint.
Why did the Founders implement such a weak, divided government? One big reason: they clearly understood that historically the greatest threat to the freedom and well-being of a people comes not from foreign enemies but instead from their own government officials, even democratically elected ones. And they understood that that threat to the citizenry was always greatest during war.
Consider the words of James Madison, the father of our Constitution: ?Of all the enemies to public liberty war is, perhaps, the most to be dreaded, because it comprises and develops the germ of every other. War is the parent of armies; from these proceed debts and taxes; and armies, and debts, and taxes are the known instruments for bringing the many under the domination of the few.?
What does our Constitution say about war? Our Founders divided war into two separate powers: Congress was given the power to declare war and the president was given the power to wage war. What that means is that under our system of government, the president cannot legally wage war against another nation in the absence of a declaration of war against that nation from Congress.
Again, reflect on the words of Madison: ?The Constitution expressly and exclusively vests in the Legislature the power of declaring a state of war [and] the power of raising armies. A delegation of such powers [to the president] would have struck, not only at the fabric of our Constitution, but at the foundation of all well organized and well checked governments. The separation of the power of declaring war from that of conducting it, is wisely contrived to exclude the danger of its being declared for the sake of its being conducted.?
Therefore, under our system of government although the president is personally convinced that war against a certain nation is just and morally right, he is nevertheless prohibited by our supreme law of the land from waging it unless he first secures a declaration of war from Congress. That was precisely why presidents Wilson and Roosevelt, who both believed that U.S. intervention in World Wars I and II was right and just, nevertheless had to wait for a congressional declaration of war before entering the conflict. And the fact that later presidents have violated the declaration-of-war requirement does not operate as a grant of power for other presidents to do the same.
What about the congressional resolution that granted President Bush the power to wage war against unnamed nations and organizations that the president determines were linked to the September 11 attacks? Doesn?t that constitute a congressional declaration of war? No, it is instead a congressional grant to the president of Caesar-like powers to wage war, a grant that the Constitution does not authorize Congress to make.
Therefore, when a U.S. president wages what might otherwise be considered a just war, if he has failed to secure a congressional declaration of war, he is waging an illegal war ? illegal from the standpoint of our own legal and governmental system. And when the American people support any such war, no matter how just and right they believe it is, they are standing not only against their own principles and heritage, not only against their own system of government and laws, but also against the only barrier standing between them and the tyranny of their own government ? the Constitution.