The British Mark I was the first ever tank to see action.Developed in 1915,the Mark I got the term "tank" from the fact that it was designed and built in such secrecy that the factory workers thought they were building water containers.When it first appeared on the battlefield,however,the term stuck,
and the Mark I soon gained a reputation as one of the most effective weapons used to break through the usually impenetrable German trenches.The Mark I proved very successful and soon more versions of the original were built,the Mark II,Mark III,Mark IV and Mark V soon rolled out and became the most feared weapon for the Germans who had very few ways of destroying them.The Mark series was used by many of the Allied countries during World War I and by both sides in the Russian civil war.
http://en.wikipedia.org/wiki/Mark_I_tank
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Tigertank