The generals who face the most daunting challenge of their lives

Lt-Gen David McKiernan is so low-key that he makes even Tommy Franks, the dry and understated head of US Central Command who is running the 2003 Gulf war, look like an ego-driven celebrity hound. Yet Lt-Gen McKiernan, who sometimes speaks so quietly you strain to hear him, is the man who commands coalition forces on the ground in Iraq.

Like General Franks, he is a soldier's soldier, steeped in the entrenched culture of the Pentagon, in style and temperament far removed from his civilian boss, the iconoclastic, swaggering Donald Rumsfeld. The Secretary of Defence has all along pressed for an unconventional war, putting a premium on speed and flexibility.

Lt-Gen McKiernan knows that "asymmetrical" warfare – in which enemies (the terrorists of 11 September, perhaps the Iraqis now) use unorthodox means to negate the overwhelming technology and firepower of American forces – can impose new rules. But he likes to do things the careful way, even it it seems old-fashioned. As he pointedly noted of the feverish US and British troop build-up in the Gulf, "I don't want 'em just in time, I want 'em a little bit early."

Lt-Gen McKiernan is widely admired for his cool head and sure grasp of tactics. As a colleague put it, "he's not a yeller, not a screamer". Rather he is a classic "army brat", born into a military family – a professional soldier who served in Germany, Korea, the 1991 Gulf War and in Bosnia, before a stint at the Pentagon in the position of Deputy Chief of Staff for Operations.

Admirers say he has a sixth sense for divining the intentions of the enemy. It is a quality that retired General John Mountcastle, Lt-Gen McKiernan's commander in Germany, callsFingerspitzengefühl,that instinct that can trump conventional thinking.

In this war, Lt-Gen McKiernan wears three hats: he is leading the Third US Army, the Army Forces Central Command and the Coalition Forces Land Component Command. In the 12 years since he was last fighting in the deserts of the Middle East, he says, "We have grown exponentially in what we call 'battle command'."

If Lt-Gen McKiernan is the man running things back at headquarters, William "Scott" Wallace, 55, is the top general in the field. He is the commander of the US Army's 5th Corps, which will lead the assault on Baghdad itself.

His command post is a tracked C-2 command vehicle, just behind the advancing US front line, now believed to be little more than 50 miles from the Iraqi capital. With his men, he is blunt about their mission: the road home goes through Baghdad. The war might not last more than a few weeks, but he warns them, "We've got no business to underestimate this enemy. He's cagey, he's foxy, and he's going to fight."

Last week, Lt-Gen Wallace predicted that President Saddam had "every intention" of using chemical weapons. What he could not judge was "whether he'll use them early and frequently, whether he'll use them as a last effort or even whether his subordinates will follow his orders if he gives them the orders to use them".

Lt-Gen Wallace is a West Point graduate and, like General McKiernan, hugely experienced. He is one of the US military's few remaining Vietnam veterans in active service.

Now he faces his toughest challenge against an enemy who is putting up more resistance than expected.