U.S. President Barack Obama addresses troops at Bagram Air Base in Kabul

Suddenly, it seems, the world is at war.

In Iraq, armed and angry militants of the Islamic State of Iraq and Syria (ISIS) are at the gates of Baghdad. In Pakistan, government forces are mounting a ferocious campaign against the Taliban in North Waziristan. In Syria, the civil war drags on. These are “hot wars” involving the clashing of troops and weapons. Having escaped such “hot” conflicts in Iraq and Afghanistan, these are the sort of war Americans have made it plain they are not prepared to fight.

But there are other wars going on. In Yemen, a forgotten war against an al Qaeda outcrop continues, largely fought with lethal U.S. drones. In Ukraine, Moscow is undermining the Kiev government by stealth. Russian President Vladimir Putin, anxious not to press his luck after successfully snatching Crimea from Kiev, is like a fox sliding through the hen coop, careful not to set off the alarm. He is being countered by targeted sanctions imposed by the United States and the European Union. These are “cold wars” — a contemporary variation on the 40-plus years of  Cold War fought to a standstill by the United States and the Soviet Union.

vietnam -- soldiersThe very nature of war has changed since the hauling down of the Berlin Wall in 1989. As the Cold War raged with often imperceptible intensity, the two sides mounted “hot wars” by proxy in minor theaters — the most prominent and punishing for the United States being Vietnam, a “cold war” first fought with teams of U.S. advisers, war materiel and money that became “hot.”

Before long, the heat became too intense for the American people and their children, who were conscripted to fight, and they called for a halt. Even so, it took many years to wind down. And when the last Americans scrambled out of Saigon, the city had already fallen to the Viet Cong and been dubbed Ho Chi Minh City.

Every U.S. war since the tragedy of Vietnam has been judged against that bruising conflict. It was even assumed for a while that Washington would never take part in a hot war again. Iraqi leader Saddam Hussein’s occupation of Kuwait in 1990, however, threatened the U.S. national interest, and President George H.W. Bush decided to take the oil-rich nation back by force. With memories of our bloody entanglement in Vietnam still ringing in his ears, Bush stopped the Gulf War a little way over the Iraq border.