Gates' trip is touted as a holiday visit to thank close to 100,000 U.S. troops serving in the war. But his observations and meetings with Afghan President Hamid Karzai as well as top U.S. military and civilian leaders will probably provide crucial last-minute guidance for a review of U.S. military policy in Afghanistan due for completion next week.
The review will gauge whether Obama's 30,000-troop surge, ordered a year ago, is working to tamp down the stubborn insurgency and train Afghans to take control of their own security.
Pentagon spokesman Geoff Morrell said Gates is "feeling pretty good" about security progress.
The unannounced visit is the 10th of Gates' Pentagon tenure. It comes close on the heels ofto Bagram last Friday. Obama's stay was curtailed by bad weather that kept him grounded at the base, and he was unable to travel to Kabul to meet with Afghan President Hamid Karzai.
In the face of a war-weary public and Congress, military leaders have voiced cautious but increased confidence that the newest strategy is working.
In his remarks to troops at Bagram, Obama cited "important progress."
There are now about 150,000 coalition forces in Afghanistan. Over half are Americans. The U.S. and its NATO partners agreed last month in Lisbon, Portugal, to begin turning over control to local Afghan authorities in 2011, with a goal of completing the transition by the end of 2014.
Gates' arrival at the battlefront follows a visit to the aircraft carrier USS Abraham Lincoln, which is deployed off the coast of Oman. Fighter jets launched off the carrier participate in air operations in Afghanistan.
The coalition has beefed up its air war, with coalition aircraft dropping 1,000 bombs and missiles in October, one of the highest monthly totals of the war.
The largest number of sorties were in eastern Afghanistan, where the coalition is targeting militant leaders, including those with the Haqqani network, a Taliban faction affiliated with al Qaeda.