It is said that novices in the military arts study tactics and masters study logistics, and more and more I am convinced that logistical problems are the most basic and essential problems that need to be solved for any effort to be successful, despite my own lack of native abilities in such matters. In few efforts are logistics more essential than in the military arts, for reasons that ought to be trivial. Soldiers need food (hungry soldiers aren’t often effective), tanks and trucks need to be fixed and need fuel to run, guns and helicopters and airplanes need ammunition. All of these are logistical matters essential to success.
For those who are not aware, Afghanistan is a landlocked country. This means it has no access to the sea, and the easiest and cheapest way, by far, to send supplies over a long distance is by ocean (assuming you don’t have a spacecraft to do it). As a practical matter, given Afghanistan’s rather unfortunate location, there are basically three logistical pathways into this benighted country, and all of them have their own problems.
The first way into the country is through Iran. Iran has a lot of influence in Afghanistan and the western part of Afghanistan, around Herat, is heavily Iranian in its culture. Since Iran is not a particularly close friend the United States, this logistical pathway is not open for American or NATO efforts in Afghanistan for understandable and obvious reasons.
The second way into the country is through Pakistan. For various reasons  , Pakistan has problems with the United States right now and (accurately) sees that American involvement with Afghanistan is going to end sooner rather than later and they wish not to have internal problems with the Taliban elements within their own borders. In addition, Pakistan is not really a genuine ally with a worldview aligned with the United States or the West in general (unlike, say, Taiwan and South Korea and Israel, all of which have very serious commitments to democracy). Pakistan has pretty sound geopolitical reasons to no longer wish to show friendliness towards us, apart from their military’s active hostility toward our forces.
And then there is the matter of Russia. The only other logistics route into Afghanistan is through Uzbekistan and Russia along Russia’s rail system. Now, Russia is threatening to remove our access to that logistical supply because of their concerns to America’s missile system being built in former Soviet-dominated Eastern European nations, who for very understandable reasons want to be protected by the West and not fall under Russia’s sphere of influence again. Of course, Russia is a rather paranoid nation (for understandable reasons) about having a lot of territory to protect its more or less indefensible heartland, given that the most effective weapon in Russia’s defense is the defense in depth that allows Russian winters to take their toll before their core regions are taken.
For very sound logistical reasons, therefore, the American effort in Afghanistan may be untenable. With an unfriendly Russia and enemies in Tehran and Islamabad, there is no other way to supply ground troops in Afghanistan. The tyranny of logistics indicates that unless one of those three nations is our friend and ally then we cannot supply troops or vehicles or have boots on the ground in Afghanistan. And the reality is that none of these three nations is our friend. What that means is that we may have to leave Afghanistan sooner than we have prepared, unless we are willing to put a lot of lives at risk because we cannot effectively feed and supply our troops there in the future if Russia and Pakistan make good on their threats. That is a sobering and unpleasant reality.