Could this be the reason

Trump wanted NATO members to increase their military spending?

As far as most of Nato’s European members who contributed troops to Afghanistan were concerned, this was a theatrical exercise designed to keep the US committed to the defence of Europe. In the case of the Poles and the people of the Baltics, the great spur here was fear of Russia. In the case of the French, it was largely because they can only maintain their sphere of influence in Africa with American help. In the case of others like the Germans, it was because they knew that they were not capable of summoning up the fighting spirit or the military spending to maintain peace in the Balkans and deter Serbian or Turkish aggression there. The memory of Europe’s 1990s failure in Bosnia runs deep. 

This makes for quite interesting reading but it is long-ish.