A word coined by the diplomatic community, the equivalent of a demand letter, strongly worded warning by one country to another and often, either explicitly or implicitly, with the threat of military consequence.
Demarches are often precursors to hostilities or war or escalating diplomatic initiatives.
In September, 1996, for example, US President Clinton issued a demarche to Iraqi President Saddam Hussein when intelligence reports showed troops massing along the border of Kurd communities.