I have discussed this in great detail in the past--more than once--and it's pretty much pointless to repeat it all again. Suffice it to say that even without Iraq, with ten times the troops in the field, and bin Laden's head on a plaque on Bush's wall, Afghanistan would still have been a long, hard slog with few if any differences from Iraq in the problems of unconventional warfare and a decentralized population.

The issue was, and still is, the Taliban/Pashtun/al Qaeda enclave that has been in western Pakistan since the beginning. Nobody has an easy answer for that, and nobody would have had one even if Afghanistan had been the only war.