From 1917: War, Peace, and Revolution, by David Stevenson (OUP Oxford, 2017), Kindle pp. 58-59:
Although two US merchant vessels had gone down, neither loss seemed unambiguously due to unrestricted submarine warfare. None the less, by late February American freighters were sailing towards the war zone, which meant ‘overt acts’ were just a matter of time, and Wilson acknowledged that only luck had so far prevented them. He was reconciled, in other words, to measures that were virtually certain to lead to shooting war, and primarily in defence of US citizens and commercial interests. What remained unclear was how far public opinion would support him, how extensive America’s participation would be, and how far it would concert with the Allies.
During the following month the answers crystallized, and in the first instance due to the Zimmermann Telegram. Its origins are inseparable from the continuing revolutionary upheaval in Mexico, in which Wilson had already twice intervened. American forces had landed at Veracruz in 1914, and the casualties had preyed on his memory, while for months during 1916 US troops had pursued Pancho Villa across the north of the country. Germany, conversely, assisted the Constitutionalist movement of President Venustiano Carranza. Zimmermann had been involved in this effort and his expertise in subversion was one reason he became foreign minister. However, the idea of a Mexican alliance came from a junior Foreign Ministry official, Hans Arthur von Kemnitz. That of linking an approach to Mexico with one to Japan also had a lineage, extending back to German–Japanese contacts in Stockholm during 1916. Zimmermann and Bethmann approved the scheme with little discussion, and Ludendorff also endorsed it. It testified to the Germans’ cynicism, as they were quite unable to give Mexico serious help and an air of the absurd hung over the enterprise. Regardless, in its finalized form on 13 January the telegram instructed the German envoy in Mexico City, Heinrich von Eckardt, to propose an alliance to Mexico as soon as American entry into the war was considered imminent; to offer financial support and German acquiescence in Mexico’s acquiring territory lost to the United States in Texas, New Mexico, and Arizona in 1848; and to suggest that Carranza invite Japan to join the combination. The telegram went to Bernstorff to forward to Eckardt, which he did on 19 January. As the British had cut the Germans’ transatlantic cables it could be sent only because the United States—ironically in the interests of facilitating peace negotiations—had permitted Germany to use American diplomatic wires. But as the British were intercepting the communications of the American embassy in London, the message came to Room 40, the decrypting and decipherment unit of the Naval Intelligence Division in the Old Admiralty Building in Whitehall. Initially the proposal was presented as a contingency plan, to be pursued if America entered the war, but in a follow-up message on 5 February Zimmermann authorized Eckardt to consult the Mexicans as soon as he thought appropriate. A partially decoded version of the initial telegram went to Admiral Sir Reginald Hall, the Director of Naval Intelligence, as early as 17 January, but Hall delayed before forwarding the information to the Foreign Office, for fear the Americans learned that Britain was reading their traffic. It was Hall’s idea that Balfour should give the decoded text on 23 February to the American ambassador, Walter Hines Page, by which stage the British had obtained a further copy in Mexico City and Balfour could obscure the real source with the half-truth that it had been ‘bought in Mexico’.
What matters here is less the telegram’s provenance than its consequences. Page reported it on 24 February. It showed that even when the Germans had seemed open to American mediation they had already decided for unrestricted submarine warfare and were plotting an anti-American alliance.