Picture
Zhuge Liang Discussing Strategy
Source : http://www.flickr.com/photos/kanegen/
The story happened during the period of the Three Kindoms. Once Cao Cao from Wei led a 200,000 strong army down to the south to wipe out the kingdoms of Wu and Shu. 

Therefore, Wu and Shu united to defend his attack. Cao ordered his men to link up the boats by iron chains to from a bridge for the Cao’s passing from the north bank of Yangtze River to the south bank.