President Trump’s decision to assassinate Qassem Soleimani accelerates perhaps the most important post-Brexit decision faced by Boris Johnson: whether to stand with the US or the EU at moments of potentially acute global crisis.
What happens next will largely be conditioned by how Iran responds and retaliates. But all Western governments are examining their options.
In particular, France’s President Macron will be working hard to forge a coordinated EU and European response, not least because through the G7 over the summer he endeavoured to engineer a dialogue between President Trump and Iran, those close to Macron say he admires Johnson.
But there are members of the government who harbour the traditional British mistrust of a strong French president.
"Macron is hell bent on his ego," one told me.
For what it is worth, my strong sense is that the British government is giving Trump the benefit of the doubt, that ministers are framing the assassination as a proportionate response to a perceived Iranian threat to US personnel and commercial interests.
But if Macron, Merkel and the EU beg to differ, Johnson will be stuck in the middle - faced with a conflict of loyalties that could have profound consequences for whether the UK is perceived to be even more detached from the continent of Europe than is the inevitable consequence of Brexit.