The upgrades are being done by individual soldiers and units as the Pentagon decides how Humvees should be changed, and follow public criticism of the Bush administration for not armoring all Humvees ahead of the war.
Nearly three years after rolling into Iraq in trucks covered in many instances only by canvas roofs, the 101st Airborne Division's 3rd Brigade is adding extra layers of armor to its Humvees.
Col. Michael Steele, the brigade's commander, said he ordered the improvements because the insurgents' roadside bombs, known to the military as "improvised explosive devices," have become bigger and harder to detect.
"The responsibility of the commander is to figure out what we need to respond to this evolving threat. The easiest, the fastest and most appropriate answer is add additional armor," Steele said.
Iraqi insurgents are also using more anti-tank mines and making bombs that can penetrate the Humvee's current armor. Among the more deadly devices are explosives shaped to funnel a blast through Humvee plating sophisticated bombs that officials suspect are being imported from neighboring countries like Iran.
Because additional armor won't always stop such explosives, one bomb destroyed an Abrams battle tank last month, for instance, a National Guard unit in Baghdad has added detection devices and other measures to protect its Humvees.