The Thin Red Line

Famous Painting of the Thin Red Line

'The Thin Red Line' is a commonly used phrase, often meaning an Army that has been spread out in a defensive position. It comes from episode in the Crimean War, that is enshrined in British History.

The Battle of Balaclava 1854 - nothing stands between the British Headquarters and 400 Russian Cavalry except for the 93rd Highlander Regiment, lead by Sir Colin Campbell.

Seeing the Russian Cavalry charging towards his position Campbell told his men ''There is no retreat from here, men. You must die where you stand'. He then formed his men into 2 firing lines - this was an uncommon practice. It was military protocol to form one's men into a Square Formation (Shown below); this prevents the Cavalry from breaking the line, using bayonets to push away the enemy. However, Campbell had a very poor opinion of Russian Cavalry and beveled he could shoot them down.
As the Russian Cavalry began it's charge, the 93rd fired 3 volleys upon them; at 600 yards, then 350, and finally at 150. The Russian Cavalry were routed. Some of the 93rd began to charge at the fleeing enemy, but were stopped when Campbell said '93rd, damn all that eagerness!'.' 

A painting of a Napoleonic Square

Later after the Battle, writing in for the Times, William H. Russel said that he could nothing between the Russian Cavalry and the base of British operations but a 'thin red streak tipped with a line of steel' - this was the 93rd, and this is where we get the condensed 'Thin Red Line'.
The defeat of the Calvary was celebrated as a victory of the common Red Coats in a dangerous and unorganised war. It was clearly down to Campbell confidence in his own men, and their willingness to follow him.