This UN Human Security Report confirms the decline in conventional warfare (PDF) over the last century. Conventional wars are increasingly rare and markedly less bloody (due to the rise of nuclear weapons and an interconnected global economy).
Wars will continue to occur, but in a new form. Future wars will be fought over systems and by networks. The participants won't (or at least, shouldn't) measure the effectiveness of their activity in body counts but rather through their impact on systems. For the West that means efficient systems (or at least resilient systems), for guerrillas it means disruption (which translates into capacity to coerce states or breakdown state function).