PARIS — They lurk in the oceans, a last resort to pulverize attackers with nuclear fire should France’s commander in chief ever make that terrible call. French President Emmanuel Macron, the person with the power to unleash France’s nuclear arsenal, will on Monday update French thinking on the potential use of warheads carried on submarines and planes, if it ever came to that. This in the context of concerns in Europe that Russian war-making could spread beyond Ukraine, and uncertainty about U.S. President Donald Trump ’s steadfastness as an ally. For decades, Europe has lived under a protective umbrella of U.S. nuclear weapons, stationed on the continent since the mid-1950s to deter the former Soviet Union and now Russia. Lately, however, some European politicians and defense analysts are questioning whether Washington can still be relied upon to use such force if needed. As the only nuclear-armed member of the 27-nation European Union, the questions are particularly pertinent for France. Possible revisions to France’s nuclear deterrence policy, sure to be carefully calibrated