Just wrote an item for next week's magazine about the relief efforts last month in Japan, and the performance of the C-17 in particular. Apparently Sendai airport was not fully open until 28 March following the tsunami on 11 March. The first C-17s, however, were going into Sendai on 20 March after the USAF's 320th special tactics squadron and JSDF personnel cleared the runway.
The 320th has an interesting role: establishing drop and landing zones, combat medical care and evacuation, as well as combat search and rescue. In a war, these would be the guys in charge of repairing airstrips following their capture by US forces. Sendai, of course, was not a combat zone, but it may as well have been: wreckage of vehicles and aircraft strewn all over the runway and apron, making fixed wing ops extremely difficult. The urgency of getting aid to disaster victims would have put a great deal of pressure on the team to get the airfield up and running.
These three videos by USAF Staff Sergeant Mark Leahy give some scope of the C-17 mission to Japan, and the type of heavy kit they're capable of getting into a key area fast.