You are mixing rhetoric into your argument. Nations make agreements between themselves and bodies like the UN, and violating these agreements can be called legal. War is not the breakdown of social order, often order continues within the societies of the belligerents. We may say that an act of war is legal if its declaration and conduct does not break any of the nation's agreements.
When a country decides to break its agreements it is more useful to say that there is no strong rule of law in international relations, rather than that there is no law.
when a country like the U.S. chooses to initiate a war of aggression against a foreign population it is effectively just an assertion that there is no such thing as law when it comes to wielding geo-political power. in the case of the congressional vote to authorize force, that wasn't really an exercise of law. that was a public performance of assent to break the law. it was congress saying "we capitulate to your plans and will not attempt to interfere with the war".