Senator Chuck Hagel spent 13 months as a lowly grunt in the Mekong Delta in the deadliest period of the Vietnam War. He saw the horror of war from the bottom up — men sheared in half by explosives, half-decapitated by sniper fire, bleeding to death in the gloomy swelter of the jungle. Thirty years later, he came to believe he had been used.

Senator John McCain was shot down 3,500 feet above Hanoi on a bombing run one month into his tour. He spent five and a half years as a prisoner of war; he was held in solitary confinement, tortured, beaten until he could not stand. An admiral’s son and a Navy pilot, he came to believe, like many pilots, that the war had been winnable, if only it had been fought right.

Memories of Vietnam haunt the public debate on the war in Iraq. They also lurk in the private thoughts of a generation in Congress — men like Senators Hagel and McCain, who lived through the earlier war, vote on the current one and, despite their shared past, now disagree profoundly on what the United States should do next.