"Cheval de bataille" (or in the plural, chevaux de bataille) is French for "battle horse" or "warhorse," but in both French and English the phrase means someone's favorite topic. This can be either the the sense of: the hobbyhorse they won't dismount from, the bug up someone's butt, the subject they just won't leave alone:
"Slim had flagrantly expounded upon his cheval de bataille, escape from imprisonment." (from a piece of fiction by Richard Ellis, copyright 2003)
or
"I left article 24 (child’s right to health) to the end because this article is unfortunately becoming 'le cheval de bataille' in all negotiations at the U.N." (a 2001 speech to the World Family Policy Forum by Amina Mesdoua)
or the less negative sense of the expression as: the argument that can always be relied on, a useful focus, the favorite thing that satisfies.
"The work was clearly a cheval de bataille and brought the house down," from an August 1999 review of a performance by Orchestra Filarmonica della Scala.
or
'So while I agree conceptually, pragmatically "free information" will probably not be our main "cheval de bataille" altought we will support any effort in that direction,' from a 2003 discussion of a Quebecois open source software organization.
Thus Roget's Thesaurus lists the phrase as a related term for both "vanity" and "plan".

It also seems to be the name of a defensive play in American Football, according to a Coaches' Handbook for the Playmaker Football game/simulation. It was also an early 1950s nickname in France for the Citroen 2CV (also known as "deux chevaux" or "two horses") automobile.

Sources:
http://wordsmith.org/awad/archives/0803
http://www.pmfb.net/handbook/defplays/cheval.shtml
http://www.proz.com/?sp=h&id=440491
http://www.worldfamilypolicycenter.org/wfpc/Forum/2001/Mesdoua.pdf
http://www.websters-online-dictionary.org/definition/english/st/stroke.html
http://www.yourthesaurus.net/makeshift.html
http://members.cox.net/2dellis/douglas/douglas_1150.html
http://www.scena.org/columns/anson/990825-PA-acis.html
http://www.up4.com/archives/000026.html
http://poets.notredame.ac.jp/Roget/880.html
http://poets.notredame.ac.jp/Roget/626.html
http://tyamlabo.geijutsu.tsukuba.ac.jp/www/ahiru/words.html

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