About the only US ground forces in the region are Special Forces and that 1,000 man group out of Fort Drum. If you're talking about an honest-to-God ground war, we'd need to move several divisions over there. It took months to build up those forces for the Gulf War and they had the advantage of a good staging area with ports. Afghanistan is landlocked and the neighboring countries aren't trilled at the prospect of us launching combat operations from their soil.

Logistics is always the key to victory, and getting serious logistical support to ground troops in Afghanistan will be a serious problem. Even giant transport planes like the C-5 and C-17 can only carry a single M-1 Abrams tank at a time. An M-1 weights about 63 tons. A C-5 could possibly carry two M-1 tanks at once but only if it took off with a low fuel supply and was escorted by at least one tanker plane. There are fewer than 100 C-5s in the inventory and not very many C-17s, either. The C-17 just entered service a few years ago and not very many have been built. A C-130 or C-141 isn't big enough to carry tanks, so they'd have to carry in troops, fuel, ammo, and equipment. Without a port or staging area, it'd be very difficult and time consuming to mount an invasion.

Special Forces troops can go in a lot easier than armored units but SF can only do so much. If it comes to a ground war, our best bet is to help opposition groups like the Northern Alliance (and hope they're better than the Taliban.) Russia has promised the NA a lot of military equipment but I don't know if any has reached the region yet. Nothing would be more difficult and yet more important to demoralizing the Taliban than if SF could mount a successful snatch mission against bin Laden. It'd be wonderful if they could snatch him from right in the middle of his forces.