The U.S. 2nd Cavalry conducts a cavalry attack, September 13, 1918.

Insignia of the 2nd Cavalry Regiment.

On this day in 1918, the 2nd Cavalry conducted a cavalry attack as part of the offensive at St. Mihiel.

The details can be found on the excellent Road To The Great War blog here:

Roads to the Great War: The AEF's Sole Cavalry Attack: The 2nd Cavalry at St. Mihiel [Ed. Note: The men of the four American cavalry regiments that were sent to France usually found themselv...

The attack was a small scale one, conducted by Troop F of the 2nd Cavalry, and was somewhat in the nature of a running cavalry patrol.  Troop F. was lead by Captain Earnest Harmon, who would go on to be a figure in the post war occupation of Germany, and then be a notable World War Two armored officer.

The attack is sometimes considered to be the only one conducted by U.S. Cavalry during World War One, which is somewhat debatable.  Indeed, it's debatable if it was a true attack, in the charge sense, at all, but it was certainly a mounted advance.  It was not, however, the only one conducted by American cavalrymen during World War One.

The 2nd Cavalry was notable in and of itself for being one of the only U.S. Cavalry regiments, and some sources say the only U.S. Cavalry regiment, to deploy in its entirety to the France during World War One. Again, that claim is debatable.  The 2nd Cavalry also provided the first troops to land on French soil during the Great War as troopers from the 2nd escorted Gen. Pershing to France.