War, and espionage seem to go hand and hand in today's world, Each side of the battlefield is desperately trying to figure out what the other side is doing, without being caught themselves. Of course today there are a wide variety of tools we have at our disposal to do just that, mostly electronically and digitally, to varying degrees of success.
Wind the clock back, oh say, a few thousand years, to the age of Rome, and one of its most notable figures, Julis Caesar who ruled the vast Roman empire in the 1st century BC, Rome was constantly under threat, both from outside and inside its borders, the need arose for Caesar to come up with a way of communicating to other people, namely military leaders, without the whole empire knowing about it. A way of encrypting messages, making them unreadable to enemies of Casear, was needed.
The Caesar Cipher was one of the most widely used of all ciphers. A cipher being defined as "an algorithm used to either encrypt or decrypt a message" from its normal version (Plain Text) into, what an enemy would read as, a series of garbled messages, that could look like nonsense or even a foreign language. Of course in the days of Rome a cipher would have to be nowhere as complex as today, mainly because the vast majority of the population was illiterate, which helps things nicely!.
Historically, a Caesar Cipher was what one would call a "Three letter shift" cipher basically stating that if one was to write down the normal English alphabet from A-Z on a piece of paper, and below it write the same alphabet only instead of starting with A, one would start with the fourth letter in the alphabet, being D and working all the way down to Z after the letter Z, A,B, and C, would follow. Instead of using the the natural letter for the word being written, the chart would be used, so the word CAT for example, would be "FDW", making the message all but invisible, and therefore useless, unless someone on the receiving end had the cipher and could translate it.
Currently we are not exactly sure how stellar the Caesar cipher performed, although there are no surviving records in history stating it was broken, until the 9th century AD, with the advent of frequency analysis, which is a way to encrypt a shifting letter cipher, by, overtime, looking at patterns in each letter or document and figuring out what each letter meant. Frequency analysis is a slow process but after a cipher is cracked it can be a huge advantage to any party wanting to get their hands on the encrypted message.
In today's world when data encryption is a part of everyday life, a simple shifting cipher is inadequate, as many computer programs have been developed simply to do millions of calculations a second to break the code. Unfortunately, humanity in its never ending quest to better itself, has taught the masses how to read, which makes life so much more difficult for those who want to have information hidden and encrypted from prying eyes.