Personally, I think it's a good thing that the bar is high for "old-fashioned declared war," especially if it is a constitutional prerequisite to deficit spending.

IANAL, but my career requires me to be very familiar with certain aspects of international law because I sometimes deal with them in a very direct and pragmatic manner. I probably know more about international law than your average domestic lawyer (but certainly much less than a lawyer who actually specializes in international law!).

Almost all international law falls into one of two categories: treaties and precedent.

Precedent is huge: if you can enforce an otherwise illegal claim for long enough, it becomes established international law. That's why the U.S. military engages in freedom of navigation operations, to prevent excessive claims to territorial waters and airspace from becoming established precedent. Likewise, if you fail to enforce your sovereign rights as a nation, you eventually establish a precedent that those rights are no longer yours, much like copyright enforcement. In short, if other nations let you get away with something for long enough, it literally becomes legitimate.

The UN is based on treaties, as are most of the laws of war. The thing about treaties is that breaking them only matters if someone else has the will and the means to stop you. A lot of people considered the U.S. invasion of Iraq to be illegal, but the U.S. did not become an "international outlaw" because, to be perfectly honest, nobody has the means to overtly punish the U.S. for anything. The U.N. security council can't take action against the U.S. because the U.S. would just veto any anti-U.S. resolution. Outside the purview of the U.N., the only options for punishing an "international outlaw" are censure, sanctions, and military action. Censure is pretty much meaningless, as PRK, Iraq, Iran, and plenty of others have demonstrated. Any sanctions against the U.S. would hurt the rest of the world economy at least as badly as they would hurt the U.S., so nobody wants that. Military action against the U.S. would most likely result in more harm to the "punishers" than to the U.S, not to mention the damage to the world economy. Basically, the U.S. can get away with anything short of launching overt wars of aggression against peer nations, because nothing else would be worth the trouble of stopping the U.S. The main deterrent against violating the laws of war is the possibility that you might lose, in which case your leadership would be subject to Nuremburg-style trials. If you're in no danger of losing, that deterrent is nonexistent. The only other external reason to follow the laws of war is a quid-pro-quo that you and your enemy will treat each other's POWs well (which, with some notable exceptions, existed on Germany's western front in WWII but not on the eastern front). Of course, there's always internal reasons to observe the law of war, such as societal morals and values, which brings me to my next point:

International law is, when you boil it all down, "might makes right" restrained by a combination of self-restraint on the part of super-powers, and the rational desire of most nations to avoid costly and destructive wars and protectionism (with regular lapses in this rationality).