This entry addresses the price of failure in Afghanistan, which is something I think gets glossed over my many who believe we should either withdraw or significantly reduce our operations and footprint in Afghanistan:
Question 1: This question is extraordinarily broad. Our allies, even if narrowed to NATO countries, for instance, have interests that will inevitably diverge with ours in multiple ways -- even on Afghanistan. The question could be addressed on so many levels, even days of informed blog debate could hardly scratch the surface. When we say interests, are we talking about resources? This opens the pandora's box of whether, to name two "interests," oil and natural gas may be more cheaply available, e.g., for European countries via a more friendly, non-Taliban-run Afghanistan. Or will Europe eventually sue for a deal with the Talibs if we leave Afghanistan, due to seeing business deals with the power-brokers as better than no deal at all? Or are we talking strategic interests in a geographic sense? Then we have to assess, for one thing, whether or how to impact the power struggle between India and Pakistan as it's playing out in Afghanistan (Pakistani strategic depth vs Indian encirclement). Not to mention what our interests are in maintaining some form of geographic hedge against Russia's desire to be unchallenged powerbrokers in their "near abroad." Or interests in a geopolitical sense? Is a "democracy" adjacent to Russia's "near abroad" in our "interests," as perhaps a way to increase influence for "democracy" in Central Asia? Or is it in our longer term interests to ensure there is a "democracy" in the heart of Central Asian, Pakistani, and even Indian militant taqfirist jihadis, against whom we would ideally want cooperation and support (covert and overt) from a US-supported, stable, state actor? These are only a few questions that come to mind on the topic of "staying in Afghanistan being in our interests." The real answer many of these questions seems to be, "It depends" (although it's hard to see how the answer isn't "yes" in some ways at least). And it depends not solely on predictable things, but on the most notoriously unpredictable things imaginable: people. One observation is probably relevant here, though: Leaving Afghanistan for the Taliban to overtake, and allowing it to return to its previous state, would have negative repercussions on several levels. Not as an immediate "existential threat" to the US (arguments centered on this term are superficial and barely scratch the surface of things), but rather, (1) as a further "demonstration effect" for imitators in the region or Middle East to emulate, (2) as an internationally perceived failure of US military power, (3) as a concession to Pakistan to continue a strategy of seeking hegemony and strategic depth in Afghanistan against India (perhaps raising the specter of another Mumbai attack igniting a hot-war between nuclear armed powers), and/or (4) as a training hub for militant, anti-US actors to refine, no longer merely "terrorist" tradecraft, but also paramilitary/insurgent tradecraft to employ elsewhere against US military forces.
Question 2: Too large a question to answer. A previous reply you posted, discussing "don't let the perfect become the enemy of the good," is apropos.
Question 3: Manpower is a significant limitation. Perhaps this limitation would best be mitigated by focusing on a big picture, strategic "economy of force" (i.e. employ more limited US forces to locations where US interests are under threat, but also where host-nation security/military apparatuses are already pro-US, honest, and respected among their populace).
Question 4: The war is in the interest of the US from standpoints outlined above. It may not be in the popular interest, however, for reasons unique to the post-modern American psyche, which has lost much of what the WWII generation gained through experiencing difficulties like the Great Depression. smime.p7s