Roadside bombs, the number one killer of U.S. troops in the Iraq and Afghan wars, is still the biggest challenge facing the military as it ferries thousands of troops and hundreds of thousands of pieces of equipment out of Iraq.

Maj. Gen. Thomas Spoehr, Deputy Commanding General for support in Iraq, says the military is taking every precaution possible on its way out of the country, but these bombs are still a risk.

"We continue to experience attacks primarily from improvised explosive devices on our routes," Spoehr told Pentagon reporters while speaking on a video conference from Baghdad on Thursday. "That trend, while it has gone down, has never gone to zero."

A steady flow of heavily armored truck convoys, 30 to 50 vehicles in length, have already moved a huge amount of equipment out of Iraq.

"We had about 2 million pieces of pieces of property," Spoehr said. "Today we're down to about 600,000 pieces of equipment, about 20,000 pieces of which are vehicles or trailers."

Spoehr also announced that "the vast majority of U.S. troops will be out of Iraq by mid-December," adding that most of the 33,000 troops in Iraq today will leave by air, many of them flying to Kuwait first.

There were over 164,000 troops in Iraq at the height of the surge in 2007. They'll all be gone by December 31, 2011 after the Obama administration announced it was unable to reach a security agreement with the Iraqis for 2012.

Spoehr said no one should get the impression this mission is "a rush to the exits."

But either way, Spoehr says, everyone is gone by December 31st.

The last people out will be who Spoehr described as the "underappreciated logisticians," the men and women responsible for counting every last man and taking inventory of every last piece of valuable American property.