NRO contributor Victor Davis Hanson is a senior fellow at the Hoover Institution. His The Savior Generals will appear in the spring from Bloomsbury Books.

War seems to come out of nowhere, like rust that suddenly pops up on iron after a storm.

Throughout history, we have seen that war can sometimes be avoided or postponed, or its effects mitigated — usually through a balance of power, alliances, and deterrence rather than supranational collective agencies. But it never seems to go away entirely.

Just as otherwise lawful suburbanites might slug it out over silly driveway boundaries, or trivial road rage can escalate into shooting violence, so nations and factions can whip themselves up to go to war — consider 1861, 1914, or 1939. Often, the pretexts for starting a war are not real shortages of land, food, or fuel, but rather perceptions — like fear, honor, and perceived self-interest.

To the ancient Greek philosophers Heraclitus and Plato, war was the father of us all, while peace was a brief parenthesis in the human experience. In the past, Americans of both parties seemed to accept that tragic fact.

After the Second World War, the United States, at great expense in blood and treasure, and often at existential danger, took on the role of protecting the free world from global Communism. After the collapse of the Soviet Union, both Democratic and Republican administrations ensured the free commerce, travel, and communications essential for the globalization boom.

Such peacekeeping assumed that there would always pop up a Manuel Noriega, Slobodan Milosevic, Saddam Hussein, or Osama bin Laden who would threaten the regional or international order. In response, the United States — often clumsily, with mixed results, and to international criticism — would either contain or eliminate the threat. Names changed, but evil remained — and as a result of U.S. vigilance the world largely prospered.

This bipartisan activist policy is coming to a close with the new "lead from behind" policy of the Obama administration...