the trump administration wants to present the arrangement as though they're doing europe a favour, rather than continuing a 70 year occupation force. the europeans should not have to pay for their own occupation.  but, why would they send blank cheques to washington when they can build their own army, anyways?

http://www.europarl.europa.eu/legislative-train/theme-europe-as-a-stronger-global-actor/file-european-defence-union