E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Cheval (French, à cheval).
Troops are arranged à cheval when they command two roads, as Wellingtons army at Waterloo, which, being at the apex of two roads, commanded that between Charleroi and Brussels, as well as that to Mons.
The Western Powers will assuredly never permit Russia to place herself again à cheval between the Ottoman empire and Persia.The Times.