In the wake of the revelation of the Zimmerman telegram, President Wilson had a problem. Large segments of the American population, most notably Irish-Americans and German-Americans, had doubts about the validity of the telegram. The Hearst newspapers claimed it was a fake cooked up by British intelligence. Incredibly in light of this, the German Foreign Secretary confirmed the validity of the telegram in an interview on March 3, 1917 when asked about it by an American journalist. Overnight, American public opinion became almost unanimous that war against German was inevitable. On March 29, 1917 Foreign Secretary Zimmerman in a speech to the Reichstag attempted to justify the telegram which only further enraged American public opinion, and solidified the status of the Zimmerman telegram as one of the greatest diplomatic blunders of all time:
I wrote no letter to General Carranza. I was not so naive. I merely addressed, by a route that appeared to me to be a safe one, instructions to our representative in Mexico.
It is being investigated how these instructions fell into the hands of the American authorities. I instructed the Minister to Mexico, in the event of war with the United States, to propose a German alliance to Mexico, and simultaneously to suggest that Japan join the alliance.
I declared expressly that, despite the submarine war, we hoped that America would maintain neutrality.
My instructions were to be carried out only after the United States declared war and a state of war supervened. I believe the instructions were absolutely loyal as regards the United States.
General Carranza would have heard nothing of it up to the present if the United States had not published the instructions which came into its hands in a way which was not unobjectionable. Our behaviour contrasts considerably with the behaviour of the Washington Government.
President Wilson after our note of January 31, 1917, which avoided all aggressiveness in tone, deemed it proper immediately to break off relations with extraordinary roughness. Our Ambassador no longer had the opportunity to explain or elucidate our attitude orally.
The United States Government thus declined to negotiate with us. On the other hand, it addressed itself immediately to all the neutral powers to induce them to join the United States and break with us.
Every unprejudiced person must see in this the hostile attitude of the American Government, which seemed to consider it right, before being at war with us, to set the entire world against us. It cannot deny us the right to seek allies when it has itself practically declared war on us.
Herr Haase [note: a German socialist] says that it caused great indignation in America. Of course, in the first instance, the affair was employed as an incitement against us. But the storm abated slowly and the calm and sensible politicians, and also the great mass of the American people, saw that there was nothing to object to in these instructions in themselves. I refer especially to the statements of Senator Underwood. Even at times newspapers felt obliged to admit regretfully that not so very much had been made out of this affair.
The Government was reproached for thinking just of Mexico and Japan. First of all, Mexico was a neighbouring State to America. If we wanted allies against America, Mexico would be the first to come into consideration. The relations between Mexico and ourselves since the time of Porfirio Diaz have been extremely friendly and trustful. The Mexicans, moreover, are known as good and efficient soldiers.
It can hardly be said that the relations between the United States and Mexico had been friendly and trustful.
But the world knows that antagonism exists between America and Japan. I maintain that these antagonisms are stronger than those which, despite the war, exist between Germany and Japan.
When I also wished to persuade Carranza that Japan should join the alliance there was nothing extraordinary in this. The relations between Japan and Mexico are long existent. The Mexicans and Japanese are of a like race and good relations exist between both countries.
When, further, the Entente press affirms that it is shameless to take away allies, such reproach must have a peculiar effect coming from powers who, like our enemies, made no scruple in taking away from us two powers and peoples with whom we were bound by treaties for more than thirty years.
The powers who desire to make pliant an old European country of culture like Greece by unparalleled and violent means cannot raise such a reproach against us.
When I thought of this alliance with Mexico and Japan I allowed myself to be guided by the consideration that our brave troops already have to fight against a superior force of enemies, and my duty is, as far as possible, to keep further enemies away from them. That Mexico and Japan suited that purpose even Herr Haase will not deny.
Thus, I considered it a patriotic duty to release those instructions, and I hold to the standpoint that I acted rightly.
Donald R. McClarey
Paul Zummo
Almost Chosen People 
I wonder whether I showed you this response I gave on Quora, to the question, why did the Germans do something as stupid as the Zimmerman telegram?
I have been thinking. At the head of German thinking, such as it was, there was, I believe, an understanding that Germany’s own actions would soon make war with America inevitable. Germany was slowly being strangled by the Allied blockade, and did not have the naval strength to break it. Pound for pound, German ships were better than British ones, as they showed at the Jutland, but the British, French, Italians, Russians, and Japanese, together, made up a naval force that simply could not be broken by anything the Germans and their Austrian allies could do. So the Germans had opted for submarine warfare, something that, to contemporaries, had seemed just as bad and criminal as the invasion of Belgium. They had already abandoned it once under American pressure, but by 1917, and in spite of the land advance into Russia, they were out of options. I don’t think it is often appreciated just how bad the German situation was already by mid-1917: the country was short of every kind of resource – the winter of of 1917 was long remembered as “turnip winter”, because the citizenry could eat nothing else – and was being outproduced by Allied heavy industry, which also had brought in that new nightmare, the tank, whose still undefined potential already kept German generals awake at night. Austria was at its last gasp; in September they informed Berlin that another Italian push would finish them. And Russia, in spite of its near-collapse, still kept huge German armies tied down merely by refusing to accept that it was beaten. The Russian republican government had been heard to say that they would fight all the way to the Urals.
The logic of returning to unlimited submarine warfare was that the economic prevalence of the Allies had to be broken, and that the only way to do so was to make the sea inaccessible to them. But that is feeble at best, since the main sources of coal, steel and foodstuffs for the Allies were on the homeland territories. What I really read there is the increasingly irrational product of baffled rage, of a sense that blow after blow only seemed to make the enemy stronger, that their sense of national and indeed racial superiority did not correspond with the fact, that a motley coalition of lesser nations was bringing the great Reich down. It was, I think, an excuse for a desire to destroy, to smash, to spread grief and horror in every way possible – no longer war, but terrorism. Only they knew, even then, that to do so would bring the last and already the greatest of great powers crashing and raging in.
I do not see this as a rational decision, but rather as the by-product of delusion and despair. The Germans could not contemplate being defeated. If America came in, then what was already menacing would become inevitable. So something had to be done to distract America and tie her down. In reality this was impossible, because no power in the world had the interest, let alone the power, to attack America. The only powerful and traditional enemy was the British Empire, and German actions had managed to bring these two frowning giants almost to each other’s arms. So German fantasy seized on Mexico, not because it was likely, but because the alternative was to accept despair; just as in the second frolic they seized on the death of Roosevelt.
Good points Fabio. I also think some Germans in the Imperial government had difficulty understanding the power of the US. The idea that Mexico would be anything but a minor inconvenience to the US in a war situation apparently never occurred to the Germans behind the Zimmerman telegram. To run a risk like this for such a nonexistent gain does indicate German delusion, and at a point where Russia about to be removed from the board gave the Central Powers their best hope for victory. Getting America involved in a war that most Americans had little interest in is a classic example of bad decision making throwing away a potential victory in a completely useless manner.