unmask one's batteries

From the revealing of artillery previously concealed from the enemy’s line of sight, immediately before opening fire.

unmask one's batteries (third-person singular simple present unmasks one's batteries, present participle unmasking one's batteries, simple past and past participle unmasked one's batteries)

1. (idiomatic, dated) To begin a decisive action, especially one that reveals one’s true strength or character.
   • 1858, William Harrison Ainsworth, Mervyn Clitheroe, volume 2, page 278:

     “Your father is fond of surprises, as you must have seen,” Cuthbert rejoined. “He won’t unmask his batteries till all is ready for action.”

   • 1877, J. H. Merle d’Aubigné, translated by William L. R. Cates, History of the Reformation in Europe in the Time of Calvin, volume 7, pages 198–99:

     The bishops, proud of this first victory, believed that a second would be easily won, and they unmasked their batteries.

   • 1882 August 26, Public Opinion, volume 42, number 1,092, page 262:

     The Evénément remarks:—“The Egyptian Question is not a French or an Anglo-French, but a European question, and the spoliation just perpetrated by England, has enlightened Europe in time. It is not wise to unmask one’s batteries too soon.”

   • 1956, Carlile Aylmer Macartney, October Fifteenth: A History of Modern Hungary, 1929–1945, volume 1, page 169:

     Meanwhile the Germans had shown that they themselves were not yet ready to unmask their batteries.

2. Used other than figuratively or idiomatically: see unmask,‎ battery.

• cross the Rubicon
• show one's cards