One of the most important stories in Anglo-American constitutional history has been the struggle over the extent of what is called the "royal prerogative". That's the term for the inherent power of the monarch in such areas as foreign affairs, warfare, law-making, etc
The history of England is in many respects the history of the gradual restriction of the unlimited power of the king and the imposition of conditions and limitations that established a separation of powers between executive, legislative, and judicial branches of government. One of the central elements of the royal prerogative was the power to make war without the approval of Parliament. Even to this day, the monarch of Great Britain has the sole authority to declare war, without the consent of the legislature
That history is essential to understanding the foundation of the United States. If you were to read the Declaration of Independence, and focus on the "long train of abuses" in that document, you'll understand that the misuse of royal prerogative was at the heart of the grievances that led to the Revolution. One of the driving principles in the Declaration, and later in the Constitution, was the need to limit the royal prerogative and to limit the power of the executive with checks and balances
Article One, Section Eight of the Constitution gives to Congress the sole authority to declare war, to raise armies and navies and to regulare them. Article Two, Section Two designates the President as the Commander in Cheif of the military, which ensured civilian control of the military, but did not give him unlimited power to make war or take other actions purely at his discretion. That principle has been upheld by the Supreme Court, for instance in the Youngstown Steel case, which overturned President Truman's seizure of steel mills during the Korean War. It has always been understood, however, that in emergency situations, the President can act to defend America against attack, even without first getting Congressional approval. That same section has also been understood to give the President very broad powers to conduct the foreign policy of the United States, including making treaties
Add to this is that the United States has signed onto the United Nations Charter, which is thus part of the "supreme law of the land" according to Article Six of the Constitution. That Charter permits nations to act in self-defense against an armed attack (Article 51) but specifically forbids "the threat or use of force against the territorial integrity or political independence of any state" (Article 2). Under the Charter, and thus under American law, the authority to used armed forces against another state is reserved to the Security Council (Chapter VII)
Over the course of our history, presidents have greatly expanded their powers over war-making. Our nation has engaged in many conflicts on Presidential decision alone, without specific Congressional approval. From time to time, Congress has tried unsuccessfully to restrain that power. In recent years, Congress has completely abdicated its authority over declaring war. With a few exceptions (e.g., the Iraq War), the United States has consistently ignored the United Nations Charter when deciding to engage in armed conflicts
Why does this matter to Catholics? It has always been an element of Catholic social teaching that nations may engage in warfare under very limited conditions. This has generally been known as the "just war" doctrine, and can be found in the Catechism of the Catholic Church, section 2309. An essential element of that doctrine is that the decision to engage in war must be made according to the laws of the nation and international law by the competent legal authorites
Those requirements have been consistently flouted by our militarized government. We have now come to a place where the President has no accountability to anyone — not Congress, the Supreme Court, or the Constitution. And so we are engaged in on-going wars in Afghanistan, Iraq, Somalia, Yemen and now Syria, all of which are being waged without any regard to the Constitutional limits on presidential authority
It is as if we never separated from Great Britain. In effect, we have a monarch with unlimited royal prerogative to wage wars on other nations. These decisions are too important to leave morality out of the calculus. As Catholics, we must bring moral principles into the debate

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