London this week has been anxious under chalky skies with prolonged drizzle, apt conditions for a nation grappling with an unruly process to (probably) depart the European Union. The debate here is largely over how Brexit will affect the political and economic relations between the UK and Europe. Largely ignored, by both Brits and Americans, is how Brexit is a tragic geopolitical loss for the US and the trans-Atlantic alliance. When the nations of Western Europe operate together they represent a quarter of the world’s gross domestic product and the world’s second-largest defense budget. But individually, none is remotely as important a US partner as they are collectively. An old saw attributed to Henry Kissinger holds that there is no way to “call Europe” and ask for help in foreign policy. Yet the steps over the past 20 years by the EU — including forming a diplomatic service and a coordinated foreign and defense policy — began the process of providing that single number to reach America’s staunchest allies. Brexit imperils those gains, pulling the second-largest economy in the European bloc out of the continental trade agreement and disconnecting it politically and militarily from the rest of Europe. Moreover, it drives a wedge between the UK and the Franco-German entente that is increasingly dominating European policies and positions. That hurts Washington’s ability to build coherent coalitions to deal with the big global problems, from censuring Iran to sanctioning Russia to dealing with climate change. For NATO specifically, there is actually some good news. The British military, which now tends to divide its participation between EU and NATO missions, will shift to the latter after a Brexit. The bad news: This would make it likely that the French and Germans would simply increase their support for EU military priorities at the expense of NATO. (Consider French President Emmanuel Macron’s misguided call for a “European army.”) Even if Brexit had a neutral effect in terms of force structure, it would psychologically weaken the alliance overall. At the darkest end of the spectrum, Brexit could accelerate the overall collapse of the “European project” and the security blanket it provides. Other forces here include the election in Italy of an anti-EU populist government; the anti-democratic governments in Poland and Hungary that have been censured by the EU; the dwindling political capital of lame-duck German Chancellor Angela Merkel; and the Yellow Jacket street protests in France that undermine the sentiments of a unified Europe. What should the US do? First, Washington should stay on the sidelines in the British debate. The Trump administration has been vaguely supportive of Brexit, implying the US would provide a new bilateral trade agreement. But, rightly, there have been no concrete steps in that direction. This is an issue that must be decided by the people of Great Britain. Finally, the White House needs to plan out the policy choices for aiding a newly isolated UK. The biggest threats are not military but economic and diplomatic. In addition to a new free-trade association, the US needs to provide Britain with a stronger bilateral military-to-military engagement policy; adapt its intelligence cooperation to make up for what the UK will lose from its former EU partners; and act as an honest broker between the UK and EU until the initial round of mutual resentment and bitterness subsides. Bloomberg View