The verdict on four years of fighting in Iraq hinges on the events of the next few months.

With the U.S. public and many politicians intensely skeptical that a changed military strategy can salvage the war, the U.S.'s new commander in Iraq, Gen. David Petraeus, must win them all over — and fast.

Petraeus takes over on the heels of the resignation of Secretary of Defense Donald Rumsfeld, the departures from Iraq of Gens. John Abizaid and George Casey, and the electoral gains of anti-war Democrats.

Despite the gloom and doom, he has arrived in Iraq with a surge of more than 20,000 American combat troops, and new theories on how to conduct counterinsurgency — involving ridding terrorists from neighborhoods, and replacing them with Iraqi and American troops to ensure public safety and the restoration of basic services.

Somehow Petraeus has to quell Sunni-Shiite sectarian violence without impinging on the autonomy of the Iraqi government. That means not just winning hearts and minds, but also disarming militias; stopping the policy of arresting and then releasing terrorists; widening the rules of engagement; and preventing jihadists from infiltrating Iraq from Iran, Saudi Arabia and Syria — and all the while giving the credit to the Iraqi military.

But has a single commander ever made much of a difference in almost instantly turning around an entire theater?

In fact, yes. The once relatively unknown Gens. Ulysses S. Grant, Curtis LeMay and George S. Patton all found renown only after replacing their failed predecessors. Indeed, in almost every war, on occasion a single general can so radically change the pulse of the battlefield that a political victory becomes possible where once the public thought it was utterly improbable.

Take, for example, the Boer War between a colonial Great Britain and the Afrikaners of South Africa. Its first year (1899) proved disastrous for British forces. Their conventional forces were ill prepared for guerilla ambushes by Afrikaner irregular sharpshooters and cavalry. But with the appointment of Lord Kitchener in 1900 came the creation of British commandos and new tactics, leading to a British victory and an eventual settlement.

Great Britain faced an even bleaker situation in the first three years of World War II. But by late summer 1942, newly appointed Gen. Bernard Montgomery had reorganized British defenses in Egypt and restored morale. He then stopped German Field Marshal Erwin Rommel at El Alamein, and criticism of Churchill for the past serial losses of Western Europe, Greece, Crete, Singapore and Tobruk ceased. Britain was soon on the offensive for good.

Korea, too, was once thought all but lost. In late November 1950, hundreds of thousands of Red Chinese overwhelmed United Nations troops and nearly drove them off the peninsula. A tired Gen. Douglas MacArthur was stunned, frustrated and, in a few months, relieved of his command.

Unfazed, his replacement, Gen. Matthew Ridgeway, restored an offensive spirit, found weaknesses in enemy tactics, and pushed the Chinese and Korean communists back north of the 38th parallel. That turnaround gave newly elected President Dwight Eisenhower leverage, which eventually he used to conclude a peace that recognized an autonomous South Korea.

During the unforeseen Arab offensive of the 1973 Yom Kippur War, Israel was stunned by an even greater surprise than the Chinese invasion of Korea. Armed with lethal new Soviet anti-tank missiles and protected by anti-aircraft batteries, Anwar Sadat's 3rd Egyptian Army stormed into the Sinai Peninsula, inflicting severe losses on the stunned Israeli Defense Forces.

Conventional wisdom called for Israeli counterattacks within the Sinai. But eccentric Gen. Ariel Sharon chose instead to cross the Suez Canal to cut off the supplies of the Egyptian army and threaten Cairo. The dynamics of the entire war were radically changed by a single general's risky gambit. As a result, Israeli forces salvaged a victory of sorts from the jaws of defeat.

Today, Iraq poses no more a dire predicament for Petraeus than the past obstacles that faced these gifted generals of prior wars — even given the fact that American manpower reserves and patience are mostly spent.

After the bleak summer of 1864, Gen. William Tecumseh Sherman saved the Union cause, and with it the Lincoln presidency, by taking Atlanta. By winter, we will see whether David Petraeus can likewise do the unexpected in Baghdad.

Victor Davis Hanson is a classicist and historian at the Hoover Institution at Stanford University. Reach him at author@victorhanson.com.