One part of it was that Zimmerman, the German leadership and high command thought that it might not necessarily mean a prompt declaration of war, as Wilson, although slowly warming to entry, had been very averse to getting involved in a European war. Another important thing to consider is that the strategic situation in 1917 was looking up for Germany, with Russia reeling from losses in 1916 and revolutionary/mutinous sentiment growing, it seemed like a real possibility that the Eastern Front might collapse. This meant freeing up manpower for a decisive breakthrough on the Western Front in the near future. Although the US had a newly developed modern arms industry and a vast manpower base, their standing Army was just over 100,000 before they mobilized the National Guard, as well as poorly trained and equipped. Total mobilization and preparation for combat against a competent, hardened German Army was a daunting task and expected to take time. The European powers fighting on the Western Front had millions of men engaged across the front, so you could probably see why Germany was not too worried about American troops making much of an impact for the foreseeable future. What could the US do if Germany knocks the Allies out of the war before American troops could arrive in France? All sides during the war envisioned a decisive breakthrough with every major offensive, with expectations for victory in months and seldom beyond a year away. Some figures in the German government were very wary of drawing the Americans into the war, while some thought it would be inconsequential. Others embraced the possibility and thought that they too would be crushed by their powerful armies, or be unable to even make it to France when all their troopships were to be sunk crossing the Atlantic. It seems that Zimmerman did not expect the American reaction to be as decisive, and confirmed the news of the telegram to let Mexico know that the offer was actually on the table, as the Mexican government was not convinced that the telegram was not a forgery. It wouldn't have been totally unreasonable to think that the Mexican Army could effectively tie up large numbers of American units, as had been the case during other seemingly mismatched engagements throughout the War. If this actually happened, the arrival of American soldiers in France would have certainly been delayed or reduced in scale. All of this would of course be trivial if Germany could force a breakthrough and win the war before then. It's easy to see with hindsight the tentative position Germany was truly in, but the time horizons in which the war was expected to be fought differed from what really happened.

Somewhat related: any recommended reading to cover this period in Mexican history, or maybe a good primer on the Mexican Revolution? In Spanish is welcome, I do need to work my Spanish literacy!

Was it that Germany thought Mexico had the capabilities to tie up the US, or was it with that there were only two possible antagonists, Canada and Mexico, and they knew that Canada was out of the question, leaving Mexico as the only possible option?