Two contemporary factors that argue for less war making stand out for me. One is technology, already being touched on elsewhere in this thread. Militaries now command destructive power that better approximates what is required to accomplish a goal, enabled by technological improvements (i.e. footage of the JDAM guiding itself through a structure's window during the Gulf conflict).

Another factor is that the age of commercial airliners and the global transport systems has made efficient global migration & trade a reality for decades now. What citizens would accept attacking their brothers and sisters in another country, attacking their trading partners with whom they make their livings? Perhaps this is now the quintessential limiting factor in making war.

Um, combined with societies becoming more sex-positive, we are edging closer to realizing the mantra Make Love, Not War!