It is my understanding that we supply our forces in Afghanistan through Pakistan, and that it would be very difficult logistically if we did not go through Pakistan.  If we can't go through Pakistan, we can't reach Afghanistan.  Of course, we can't go through Iran, which would be the only other alternative.   Take a look at the map.  

So, if the Pakistanis say, fine, if you're not going to send us any aid, we will align ourselves with China, what then?   We pull out of Afghanistan?   Because we cannot supply ourselves in Afghanistan without going through Pakistan. 

And then, if we cannot put any forces in Afghanistan, do we leave Afghanistan to the terrorists- ISIS, Al Qaeda, whoever?   And at that point, we have Pakistan under threat from crazy people in Afghanistan, and Pakistan has a whole lot of nuclear weapons.   The border between Pakistan and Afghanistan is impossible to defend or seal off.  

I assume we've thought this through...

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