Conservative Position
The Trump administration asked the NATO members to contribute the amounts they already pledge to defend themselves and asked for an increase in that amount from 2% to 4% of GDP. The thinking was that in 1949 when the NATO treaty was created, these European countries were devastated financially as a result of WW II and could only contribute 2% of their GDP. 65 years later when their economies are stable and thriving these countries should be able to contribute more for their own defense.
At the NATO summit, NATO members agreed to pay at least what they originally pledged. The result was that many U.S. troops could return home, and we would spend less tax dollars on defending these countries. By Trump forcing these countries to live up to their promise it showed that America can not be taken advantage of it and it improved our standing in the world.
This was a Trump Promise during the campaign that was fulfilled.