AT 6.24AM on November 9, 1914, the Anzacs commanders' worst fears came true as they sailed in convoy to war.

As the convoy was within about 80km of the Cocos Islands and had passed the night in a dark, relative calm, wireless operators on many of the ships suddenly received a short, loud message they did not recognise. The signal was heard twice more over the next few minutes.

The next communication came from the Cocos Islands telegraph post: "SOS. Strange warship approaching."

Captain Mortimer L'Estrange Silver, commander of convoy flagship HMAS Melbourne, signalled to HMAS Sydney to leave its protective position by the troop ships and make for the Cocos Islands to investigate the threat, which was believed to be the German cruiser Emden.

According to official war historian Charles Bean, within an hour the Sydney had disappeared over the horizon to the west.

"The whole fleet had seen the Sydney leave, and every man knew that she had gone to meet some ship of the enemy," Bean wrote.

By this point, the Emden had built itself quite the reputation - as much in Britain and Australia as in Germany. From September to November 1914, it had sunk or captured 23 vessels and disrupted trade over a wide area of the Indian Ocean, and many allied ships were engaged in the hunt for it.

That morning, the Emden was raiding the Cocos Islands with the aim of destroying the wireless station there and cutting the undersea cable (which was one of only two telegraphic links between Australia and the rest of the world).

The Emden's captain had no idea the Anzac convoy was so close, having assumed it would have taken more of a direct route to Europe. Ironically, the convoy had opted for the more circuitous Cocos Islands route because it was felt the more regular, direct passage might also have been more closely watched.

The battle was all over in less than two hours.

Read more here:

The Battle of the Cocos Islands