The Taliban were quickly driven from power by a joint effort by the U.S. military, the CIA and the Afghan Northern Alliance.
Then it was a matter of hunting down the remnants of al-Qaeda as the British Royal Marines, along with British special forces, pursued them over the mountains, but many fled to safety to regroup in Pakistan.
It wasn't until ten years later that commandos from the U.S. Navy's Seal Team Six located al-Qaeda leader Osama bin Laden in a villa in Abbottabad, Pakistan.
The first two years of the US-led so-called “Operation Enduring Freedom” were relatively quiet. In late 2003, as America's attention turned to Iraq, U.S. soldiers we met even began referring to Afghanistan as “Op Forgotten.” But it was still dangerous.
From a rain-soaked air base in Kandahar, we watched as Romanian troops patrolled nervously in their Soviet-era armored vehicles, fearing the next ambush.
My BBC team and I flew in a Blackhawk helicopter to a remote US-manned fire base in the mountainous Paktika province. There we were cheerfully told: “You have arrived in the worst place in the world.”
In fact, after dark, the Taliban fired Chinese-made rockets at the base, which we were told had been planted there by farmers who had either been bribed or coerced into doing so.
Everything changed after 2006, when the United Kingdom sent troops to Helmand province, a previously relatively peaceful part of Afghanistan.
The Taliban made their intentions clear. If you come, they said, we will fight you.
And yet the British government of the day seemed shocked at the intensity of the fighting in which 3 Para were now involved, with British paratroopers firing mortar and artillery fire so close to their positions that it was described as “danger close” to prevent their bases from being overrun.