Battle ensign A battle ensign is the name given to a large war flag which is flown on a warship ' s mast just before going into battle . The flag identified the allegiance of the ship in what could be a very confusing situation , with thick clouds of gunsmoke obscuring the ships in action , hence the large size of these ensigns typical for the Royal Navy from the 17th to 18th centuries of about 20 by 40 ft ( 6 . 1 by 12 . 2 m ) . It was commonly accepted that so long as a ship flew its ensign it was an active participant in battle ; prior to action it was an acceptable ruse to fly a false flag . If a ship surrendered then it would take down its ensigns ( which was known as striking the colors ) . This is also the origin of the phrase " To nail one ' s colours to the mast " , showing a determination to fight on and never surrender . In practice , warships would fly more than one battle ensign , so that if the flag was destroyed or brought down during the fighting there would be no confusion . Conversely , keeping the flag flying even though the ship might appear to be past fighting was a sign of determination rather than foolishness . The German battleship " Bismarck " continued to fly its battle ensign even after its gun batteries had all been silenced by British shells , and it sank with the ensign still up . The battle ensign was seen as an important element for the morale of the crew and was held in high regard . If a warship was sinking and had to be abandoned , flags such as the battle ensigns would be taken off the ship before it sank and were entrusted to the senior ( surviving ) officer . Some countries use their national flag as the battle ensign , while others use their naval ensign . Sometimes unique flags were made and used as battle ensigns , for example the one flown by United States Naval Forces at the Battle of Lake Erie in the War of 1812 . The battle ensign is sometimes also flown by U . S . Navy warships as a courtesy when entering or leaving foreign ports . Examples . In the United States Navy , battle ensigns from American warships and battle ensigns captured from enemy ships are displayed at the United States Naval Academy at Annapolis , Maryland . The battle ensign flown from USS " Arizona " during the attack on Pearl Harbor by the Japanese Naval Air Forces on December 7 , 1941 was badly stained with oil and it was burned as being ' unfit for further use ' , before anyone thought to save it . The USS Ramage uses the flag of the New York City Police Department as its battle ensign in commemoration of the actions of the NYPD during the September 11 attacks . The British National Maritime Museum has in its collection a battle ensign taken from the Spanish " San Ildefonso " , captured at the Battle of Trafalgar which measures 9 . 8 by 14 . 4 m ( 32 . 2 by 47 . 2 ft ) . An ensign from the French " Généreux " of 8 . 2 by 16 m ( 26 . 9 by 52 . 5 ft ) was captured by HMS " Foudroyant " in 1800 , and presented by its captain , one time Flag Captain of Lord Nelson , to the City of Norwich . On 9 November 1914 , wore a large Australian National Flag as a battle ensign from the mainmast during her celebrated victory over German warship SMS " Emden " at the Battle of Cocos . The battle was the first naval clash and first victory for the Royal Australian Navy . In the early days of World War II Captain F . S . Bell of HMS " Exeter " gave the order to hoist battle ensigns as Exeter turned to engage German " pocket battleship " " Admiral Graf Spee " , which significantly outgunned her , during the Battle of the River Plate .