The 1929 Kellogg-Briand pact technically "outlaws" war for its signatories; the Charter of the United Nations commits its members to refrain from the use or even the threat of the use of force in their relations with other states. Technically, the United States has been in full compliance with this provision, because the last war that was formally declared by Congress was America's entry into the Second World War in December 1941. Since then, although U.S. forces have seen combat, on average every four years, the United States has only engaged in police actions, interventions, support operations or humanitarian interventions. Moreover, ever since the passage of the War Powers Act in 1973, a piece of legislation whose constitutionality has never been tested, U.S. presidents have further attempted to find ways to dispatch U.S. forces into combat without triggering its provisions. The latest effort, undertaken by the Obama administration in 2011 during the Libya operation, was to argue that military action which did not involve U.S. ground forces and for which the possibility of casualties was slim, did not rise to the level of an armed conflict that would require Congressional scrutiny.
I raise this question because watching Russia's actions it is clear that Moscow too finds ways around the "no war" provision. Forces are sent into operations as part of humanitarian interventions, or in self-defense, or, in the case of the paratroopers who were captured by Ukrainian military forces, the argument is put forward that they "got lost" or "crossed the border by accident." We have the phenomenon of two countries' leaders conferring about peace in Minsk (Presidents Putin and Poroshenko) while their militaries are trading fire--but with plausible deniability that any sort of armed engagement is taking place.
The "no war" provisions of the 20th century were intended as efforts to stop armed conflict altogether. Those efforts, as well intentioned as they were, have failed. Armed conflict between states still occurs. Would we be better off if we returned to a pre-modern honesty, that forces states to declare war and in so doing to be bound by the rules of war?