Wars are intiiated by the side that believes the conflict will have a useful outcome. Often they do, examples being the American and French Revolutions, the expansion of the Roman and British empires, and the Communist takeover of China. No moral judgements here, but the methods worked. Wars are maintained as long as both sides disagree about the outcome, and are ended when one side has its will or ability to resist broken by force. War is nasty, but it works, and as long as surrender is worse than fighting to victory nations will try to win. Pacifism is no defense, although heavily armed neutrality worked well for Sweden and Switzerland. America has done rather well from most of its wars, netting independence, territory, national economic development and prosperity, not to mention self-preservation. That democracy exists anywhere is largely due to American warfighting ability, and our nuclear deterrent. BTW I don't see war as a threat to the human race, barring a really comprehensive nuclear holocaust. All the wars, natural disasters, and pandemics in the past century haven't come close to defusing the "population bomb".