The most striking moment of Turkey’s lira drama so far came, as so often in crises these days, courtesy of Twitter. President Donald Trump tweeted on Friday that he was doubling US tariffs on Turkish steel and aluminium “as their currency?.?.?.?slides very rapidly against our very strong dollar!” In past financial crises, this was exactly the time when the US Treasury might have stepped in with words or actions aimed at calming markets. Mr Trump seemed instead to revel in fuelling the fire.

Washington’s imposition of sanctions over Turkey’s detention of a US pastor, Andrew Brunson — initially against two Ankara government ministers— has triggered one of the most serious rifts of recent years with a Nato ally. It highlights too how the use, and sometimes misuse, of sanctions and punitive tariffs by this US administration risks undermining America’s own interests and those of key allies.

Sanctions can play an important role in modern diplomacy. They provide a means of exerting pressure while stopping short of armed force. The Trump White House has focused on them in part as a way to avoid the military entanglements of previous administrations. For maximum effectiveness, however, sanctions need to be employed judiciously, with clear goals, and in close co-ordination with allies.