Sentence:  The order in <WSD>echelons</WSD> is favorable for attack, because it readily conforms to the nature of the ground, and does not necessitate engaging more than a part of the forces; it is adopted for the purpose of attacking a particular point of the enemy's line. 
Meaning of the word marked with <WSD> tags:  A formation of troops, ships, etc., in diagonal parallel rows. (from late 18th c.)