The subject was some unfortunate phraseology employed by that august organ in describing the Baltic states as "ex-Soviet".
LSM has reported repeatedly in the past about the unintentional offence such slips of the tongue or pen can cause to some in the Baltic states and indeed the ambassadors' letter is couched in the politest terms, as one would expect for a missive in the language that all reasonable people still regard as the language of international diplomacy.
The ambassadors' ire was provoked by an article from Anne-Francoise Hivert about information war waged by Russia in Finland and the Baltic states.
As the letter points out, the Baltic states were occupied by the Soviet Union and never formed a voluntary part of that happily defunct institution.
Time that Le Monde stopped calling Estonia,Latvia,Lithuania 'former Soviet countries'. Letter of 3 Baltic Ambassadors sent to Editor pic.twitter.com/y69vuHUxlD— Imants Liegis (@imantsliegis) September 7, 2017
For something along similar lines, you might like to read this story we produced earlier this year when a similarly stiff epistle was delivered to Berlin.