Article by Greg Allwood

What would you have done?

At the height of the First World War, Royal Naval intelligence intercepted a message containing the following code:

"22284 22200 19452 21589 67893 5569 13918 8958 12137... 13850 12224 6929 14991 7382 15857 67893 14218 36477 5870 17553 67893 5870 5454 16102 15217 22801 17138..."

Since German transatlantic cables had been dredged and cut by the Royal Navy in 1914, the Germans had been forced to use lines from neutral Sweden or the US to communicate with diplomats and foreign governments.

The Americans let them do so to facilitate diplomacy, something very much in line with President Woodrow Wilson's objective of helping to end the Great War through negotiation.

no man's land

But all messages were to be sent 'in the open' - that is, uncoded.

So the fact that this particular message had been sent in code, and the Germans had sought and received special permission to do so, must have raised flags with British intelligence.

The man in charge of cracking the message was Nigel de Grey, a codebreaker at the Royal Navy's Room 40.

The organisation had proved invaluable throughout the war, feeding key information from cracked German messages to the Royal Navy and the government.

It's what had allowed the Navy to intercept and engage the Germans at Jutland in 1916.

guns ship

Now, de Grey would reveal these obscure cryptographic chunks for what they really were: Diplomatic dynamite.

As he patiently chipped away at the message, the words and their explosive political implications must have leapt off the page at him:

"MAKE WAR TOGETHER MAKE PEACE TOGETHER... Mexico is to reconquer the lost territory in Texas, New Mexico, and Arizona".

In a nutshell, German foreign minister Arthur Zimmerman was instructing his representative in Mexico to get the country to go to war with America.

Mexico had lost three states to the US in the Mexican-American War of 1848, and an effort to reincorporate them through war in 1917 would tie up American forces and leave the Germans free to win the war in Europe.

Officially the US had no quarrel with Germany.

Their ships had been sunk by German U-boats, most infamously the Lusitania in 1915, but the two countries had not become enemies. President Wilson campaigned and won his election on a platform of staying out of the war in 1916.

guns british

But the Germans were getting desperate. Despite successes on the Eastern front under Hindenburg and Ludendorf, Erich von Falkenhayn's gamble in the West at Verdun the previous year had failed, and the noose around Germany was being slowly tightened by the Royal Navy.

There was such a shortage of food and supplies that during the bitterly cold 'turnip winter' of 1916-17, ersatz (substitute) foods had got ever shorter in supply and ever more repulsive, while soap almost entirely disappeared.

Only a black market kept some more fortunate Germans marginally better fed.

Hundreds of thousands would be dead from starvation by the end of the war, suffering privations far worse than allied civilians.

Germany had to strike back, and that meant escalating the U-boat campaign, which was eventually expected to bring the US into the war on Britain's side anyway.

In other words, from the German perspective, the Zimmerman telegram wasn't that radical a document but for the British it was a golden opportunity, precisely because they knew how inflammatory it would be for German-US relations.

The British, though, were immediately faced with a dilemma: They had rock-solid evidence of what the Americans would certainly see as German political duplicity - but revealing it might backfire.

To start with, this would risk alerting the Germans to the fact that Room 40 had cracked their codes and were monitoring every bit of their cable traffic, causing them to change communication methods and codes.

Likewise, such a revelation would show that Britain had been effectively spying on the US.

American annoyance might manifest as a general cynicism about Europe that increased their isolationist position instead of weakening it.

The British stalled and tried to invent a cover story.

They had an agent in Mexico dubbed "Mr H" (later claimed to be a Sir Thomas Hohler) track down the Western Union telegraph office, where they knew Zimmerman's message would have been received.

He was to bribe an employee there for a copy, giving the British another method at least by which they could claim to have picked up the message, that didn't involve spying on the country they needed as an ally.

Fortunately for the British, Zimmerman's message had been sent using an older (13040 prefix) cypher.

s, if the Germans did realise there had been a code breach, they still wouldn't realise that the British actually had the most up-to-date codes cracked as well.

British tommies

In any case, weighed against the prospect of being able to rope the US into the war, it seemed like the risk was worth it.

The 13040 code, once handed over to the Americans, could be cross-referenced with the record of Zimmerman's communique coming through their wires, showing that what the British had obtained from the Western Union in Mexico was not a forgery (as anti-British elements in the US would later claim).

As things turned out, the public story that only divulged Britain's theft of the message in Mexico worked in conjunction with the Germans' solid conviction that their codes could not possibly have been cracked. They even ordered a witch-hunt for a traitor at the embassy in Mexico, believing the British must have had help.

With things going so much against the Germans they saw little point in hiding what had happened, and Zimmerman admitted publically in the German legislature that he had sent the cable.

The British were ecstatic, having not had to show their hand and still garnered full American support when they declared war in April 1917.

The Germans had sealed their own fate, and Britain and France would reap the benefit of American entry into the war, just as they later would in 1941 when Japan attacked Pearl Harbor and brought them into World War Two.

More: 'The Military Men Who Took Us To The Moon'

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