The two divisions, totaling nearly 22,000 men, were massed on the east bank of the river. With their superior numbers, arms and veteran officers, not to mention a long tradition of battlefield triumphs, they were confident of routing the ragtag band of rebels hiding in the woods and marshes on the other bank. The signal was given, and the first artillery volley fired. The soldiers moved out, crossed the river—and marched into military history. Within three days, the two divisions were annihilated, and their commander’s head was severed and sent back across the lines as a message: Don’t come back. This was not a battle from the worst days of the wars in Afghanistan or Iraq. It was the Battle of the Teutoburg Forest, nine years after the birth of Christ, in what is now northwestern Germany. It has been called “the battle that changed the course of history,” because it marked forever the limits of the Roman Empire. Latin would never take root east of the Rhine. Nearly 2,000 years later, America crossed its own Rhine of sorts, in Vietnam. Like the Romans, the U.S. military seemed virtually unbeatable, until it ventured into Southeast Asia. And like Rome’s legions after 9 A.D., the U.S. Army would recover from the Indochina debacle, retool and fight again. But its political leaders failed to learn much from the expensive overreach in Vietnam, and soon losses and costly wins became more common than decisive victories. To be sure, the United States won the Cold War without battling Soviet troops. But since its humiliating defeat in Vietnam, America has engaged in a string of significant military conflicts and emerged the clear winner in only two—ousting Saddam Hussein from Kuwait in 1991 and bombing Serbia to the negotiating table in 1995. More recently, even quick, dramatic triumphs in Iraq and Afghanistan have turned into grinding guerrilla wars, the seeds of which sprouted into the Islamic State, or ISIS. So on the 40th anniversary of Saigon’s collapse, it seems timely to ask: Can America win a war?