Sentence:  The troops selected by his Royal Highness for this daring exploit, consisted of the war battalions of the 3d, 6th, and 7th regiments of the Royal Guard, forming the first <WSD>echelon</WSD>, ... 
Meaning of the word marked with <WSD> tags:  A formation of troops, ships, etc., in diagonal parallel rows. (from late 18th c.)