LawVol:

With respect to the US, three branches are potentially involved in deciding constitutional and international law issues. With respect to decisions to go to war (jus ad bellum), the Supreme Court has made it clear that it won't get involved in second guessing those political questions, leaving them to Congress and the Executive. If the Executive acts and Congress does not act, the Executive's actions will stand (in practical effect, will be "legal"), regardless of what you, I and the woman down the street think of them. UNLESS, and this may or may not be a big "unless", UNLESS the people then take action to cause the Executive to change course.

In the case of the UN, the SC is the Final Decider as to "peace", "international security" - and to Chapter VII actions. Given the concurrence of the permanent members and the acquiescence (or silence) of the General Assembly, the SC actions will be the "law", regardless of what you, I and the woman down the street think of them.

While the velvet glove has been used by the UN versus the iron fist in Chapter VII matters, the latter is available under Arts. 48 & 49 to a greater extent than, say, under Art. 5 of NATO:

Article 48

1. The action required to carry out the decisions of the Security Council for the maintenance of international peace and security shall be taken by all the Members of the United Nations or by some of them, as the Security Council may determine.

2. Such decisions shall be carried out by the Members of the United Nations directly and through their action in the appropriate international agencies of which they are members.

Article 49

The Members of the United Nations shall join in affording mutual assistance in carrying out the measures decided upon by the Security Council.
Of course, if you are a permanent member (or a functional equivalent, such as Germany), you have less to fear from Mr Hiss' well-drafted arrangement.

Like Zhivago, you might say to the UNSC: "That only gives you the Power, it doesn't give you the Right." But, I find that of little comfort.

Regards

Mike