1580s, from French bataillon (16c.), from Italian battaglione "battle squadron," from diminutive of Vulgar Latin *battalia "battle," from Latin bauttere "to beat" (see batter (v.)). The specific sense of "part of a regiment" is from 1708. The oft-repeated quote "God is on the side of the largest battalions" (with many variants) usually is attributed to 17c. French military genius and marshal Turenne:
Madame, lui répondit-il, ne vous y fiez pas: j'ay tôujours vû Dieu do coté des gros Batallions. [E.Boursault, 1702]
c. 1300, "to contend with blows or arguments," from Old French batre "to hit, beat, strike" (11c., Modern French battre), from Late Latin battere, from Latin batuere "to beat, knock" (see batter (v.)). In falconry, "to beat the wings impatiently and flutter away from the perch." The figurative sense of "flutter downward" is attested from 1580s.