Aside from the political tradeoffs involved with increasing manpower in Afghanistan--a decision I believe is important, increasingly urgent, and should be undertaken--may well loom a larger issue concerning the U.S. military's fundamental assumption concerning the role of technology. If so, there are real gaps in U.S. power that need to be addressed.
In recent years, at least back to the 1990s, the increasingly de facto operating assumption was that manpower had been rendered less important on account of new technologies. Bolstering that assumption was the experience in the air war against Serbia during the ethnic war in and around its Kosovo region.
In Iraq, General Tommy Franks (with the strong support of Secretary of Defense Donald Rumsfeld) assumed that a "light" strategy would accomplish the military's objectives in bringing about regime change in Iraq and providing post-combat security. That assumption, much as General Eric Shinseki expected, proved disastrously wrong, and it was not until the troop surge that things were turned around. Now, a similar situation appears to be unfolding in Afghanistan, even if the specific dynamics differ.
In effect, the Kosovo experience may well have been the exception. The reality may well remain that manpower is the most important ingredient of maintaining the conventional power necessary for victory, though the breakdown of such manpower e.g., heavy units vs. special forces, would need to vary from conflict to conflict based on needs, circumstances, challenges, and opportunities. As such, a more refined military doctrine would recognize that while technology can augment manpower and its capabilities, it cannot substantially eliminate the need for manpower. That, of course, would raise questions as to how the U.S. can substantially expand its combat forces within the framework of voluntary enlistment not to mention significant public discontent over the Iraq war and growing dissatisfaction concerning the war in Afghanistan.