The Taliban were never truly rooted out of Afghanistan's hills and now, five years after the euphoric liberation of the region, they are better armed and organized into large-scale units to fight coalition forces.
Watch "World News Tonight" this week for more of Jim Sciutto's reporting on the challenges facing Afghanistan.
The response by the United States and its allies to the most recent rise of the Taliban is the largest operation since the invasion, with 11,000 coalition and Afghan troops.
"We are going into valleys, into areas where the enemy has operated with impunity before. And we are putting pressure on them," said Maj. Gen. Benjamin Freakley.
ABC News visited the U.S. base in Ghazni, just south of Kabul. It was once a quiet province, but now attacks have jumped 600 percent and the Taliban have increased their ranks from two dozen to several hundred.
During our visit, commanders followed a suspicious convoy from here to a compound just next to the base. They are always on alert.
The stand-off we witnessed ended calmly but the threat is very real, as is clear from the sight of three Humvee's destroyed in a single Taliban ambush.
Learning from Iraq
The Taliban are copying Iraqi insurgents with disturbing success. There were roughly 1,000 roadside bombs in the past year and 40 suicide bombings in the last nine months.
They are getting help from abroad, including al Qaeda bomb-makers and Arab fighters, but what is more worrisome to the coalition troops here is that the Taliban are getting the assistance of Afghans themselves.
What started out as a seemingly routine visit by one British unit to a village turned into a six-hour firefight. As soon as the soldiers left, the Taliban were back and were accepted by the villagers.
The holy war is "right here in front of our homes," said Taliban commander Ahmed Shah, "even if we're too poor to fight in Iraq or America."
At each base we visited, local commanders asked for more troops. But increasing the number of troops in Afghanistan is not the current U.S. strategy.
"I believe we have an adequate force to accomplish the mission and also not send the signal to the Afghans that we are garrisoning their country … whenever they look, they feel they are occupied," Freakley said.
It is a fight to keep control of the country and one the commanders here say they must win.