The EU, as a geopolitical entity, has an undeniable security dependency on Nato.
Yet we currently have an EU community that now discriminates against American exports.
Plus we know that the U.S. currently contributes disproportionately to western security via Nato.
Trump is, quite naturally, not happy about this arrangement.
The Republic of Ireland is a non-Nato member, inside the EU, which is a Nato member state, so to have Germany allow non-Nato Ireland to set parameters for the EU's future relationships with Britain sets a geopolitical gamble. The border issue has been the primary political obstacle to an orderly Brexit
Only if the EU can provide its own security via a renewed German military commitment would this dilemma diminish.
President Trump's rhetorical attacks on the EU and threats to withdraw from Nato create an acute incentive for change.
Germany has accumulated too many conflicting commitments to decide between them without disruption to the EU.