I absolutely get why Trump is frustrated with the war in Afghanistan. The Taliban, even if they are more fragmented than we tend to appreciate, are doing quite well, and the Afghanistan government is not performing well despite the departure of Karzai. When Obama spent much of 2009 considering whether to surge or not, I was most ambivalent for many reasons. Was Afghanistan similar enough to Iraq (where the surge seemed to work)? Wasn't the primary challenge political and not military?
So, I see where Trump is coming from. Of course, his reasoning and his analogies are flawed (Afghanistan ain't a restaurant). And that gets to the big problem now: he is uniquely unsuited to come up with an alternative policy. He has a short attention span and hates to listen to bad news. Afghanistan requires focus and a willingness to see both progress and falling backwards. Trump has allowed/encouraged Tillerson to gut State, when, again, the primary challenge in Afghanistan is political: not just about improving governance by Ghani and his administration but also figuring how to negotiate with the Taliban AND how to get the various outside actors (Pakistan primarily but also Russia, China, Iran and India) to coordinate enough to provide a conducive environment.
I have no idea if General Nicholson should be fired. I do know that replacing generals every year or so has not been good for the war effort as each one has a different strategy. This means that no strategy is really ever implemented fully, that the folks in the ground get whipsawed by the changes in rules and priorities.
On the other hand, Mattis arguing that we are losing because we do not have the right strategy may seem to miss the point. As a former general, he sees the key to winning and losing to be about getting the right strategy--the right set of plans that have various lines of effort coordinated to reach a desired endstate (yep, that is how they speak). Endstate means goal or final desired outcome. But is it about picking the right set of plans? Or is it that we outsiders have, dare I say it, limited influence? That the actors on the ground have more at stake, longer time horizons, and more influence?
Whatever strategy the US and its allies choose, the folks on the ground will be deciding whether to bet their lives on the Afghan government, on the Taliban or on staying on the fence. It is not clear that we can affect those decisions that much. We didn't influence them that much when there were more than a hundred thousand troops on the ground, so why expect more influence now. Indeed, Jason Lyall's work seems to suggest that we are damned no matter what we do. The outsiders get blamed for what the Taliban does. Oh crap.
Until we have some humility about what the outsiders can do, no strategy is going to be "the right strategy." So, yeah, Trump should be frustrated. But he lacks the capacity to think long and hard and reality-based. Thus, I don't expect a significant improvement. Perhaps he will call for the end to the US effort there, but addressing that is a blog post for another day.