From Obama's December 1, 2009 speech at West Point announcing he will raise the US troop level in Afghanistan to about 100,000 in support of his new strategy (which frankly doesn't sound very new):
First, there are those who suggest that Afghanistan is another Vietnam. They argue that it cannot be stabilized, and we're better off cutting our losses and rapidly withdrawing. I believe this argument depends on a false reading of history. Unlike Vietnam, we are joined by a broad coalition of 43 nations that recognizes the legitimacy of our action. Unlike Vietnam, we are not facing a broad-based popular insurgency. And most importantly, unlike Vietnam, the American people were viciously attacked from Afghanistan, and remain a target for those same extremists who are plotting along its border. To abandon this area now -- and to rely only on efforts against al Qaeda from a distance -- would significantly hamper our ability to keep the pressure on al Qaeda, and create an unacceptable risk of additional attacks on our homeland and our allies.
I don't get the relevance of the broad coalition point. It wasn't the absence of a broad coalition that caused failure in Vietnam; we failed because half a million American troops couldn't control the place. Further, most of the support for our presence in Afghanistan is only lip service. There were 48 nations in the "coalition of the willing" that "supported" our 2003 invasion of Iraq. Most of them were more trouble than they were worth, and almost all withdrew long before we were ready to leave.
The notion that there is not a broad-based popular insurgency in Afghanistan seems just wrong. Whatever we face in Afghanistan is almost 100% local popular insurgency. I frequently see reports that US officials estimate there are fewer than 100 Al Qaeda types in Afghanistan. On the other hand, an important similarity between Vietnam and Afghanistan is that in both places the enemy did/can retreat to and operate from another nation that was/is no-go for us.
The third point, that we were attacked from Afghanistan but not from Vietnam, is certainly correct, but what's the importance of that? We have had our vengeance on many of the Al Qaeda leadership and operatives who were involved in the attack, and it is well understood that the rest are now elsewhere, probably Pakistan. From there, they could possibly return to Afghanistan if the Taliban took over, but they could probably as well move on to Somalia, Sudan, or some other failed state we are determined not to invade. Nevertheless, tenuous as it is, this is the only thing that sounds like a real reason for our involvement—we will have 100,000 American troops in Afghanistan to keep hundreds or perhaps a few thousands of Al Qaeda bottled up in the mountains of Pakistan. This is Mr. Obama's war.