From Wikipedia: The Zimmermann Telegram was part of an effort carried out by the Germans to postpone the transportation of supplies and other war materials from the United States to the Allies of World War I that were at war with Germany.[2] The main purpose of the telegram was to make the Mexican government declare war on the United States in hopes of tying down American forces and slowing the export of American arms.[3] The German High Command believed they would be able to defeat the British and French on the Western Front and strangle Britain with unrestricted submarine warfare before American forces could be trained and shipped to Europe in sufficient numbers to aid the Allies. The Germans were encouraged by their successes on the Eastern Front into believing that they would be able to divert large numbers of troops to the Western Front in support of their goals.

Mexican President Venustiano Carranza assigned a military commission to assess the feasibility of the Mexican takeover of their former territories contemplated by Germany.[4] The general concluded that it would be neither possible nor even desirable to attempt such an enterprise for the following reasons:

The United States was far stronger militarily than Mexico was. No serious scenarios existed under which Mexico could win a war against the United States. Germany's promises of "generous financial support" were very unreliable. The German government had already informed Carranza in June 1916 that they were unable to provide the necessary gold needed to stock a completely independent Mexican national bank.[5] Even if Mexico received financial support, the arms, ammunition, and other needed war supplies would presumably have to be purchased from the ABC nations (Argentina, Brazil, and Chile), which would strain relations with them, as explained below. Even if by some chance Mexico had the military means to win a conflict against the United States and reclaim the territories in question, Mexico would have severe difficulty accommodating a large English-speaking population that was better supplied with arms than most populations. Other foreign relations were at stake. The ABC nations organized the Niagara Falls peace conference in 1914 to avoid a full-scale war between the United States and Mexico over the United States occupation of Veracruz. If Mexico were to enter war against the United States, it would strain relations with those nations. The Carranza government was recognized de jure by the United States on 31 August 1917 as a direct consequence of the Zimmermann telegram, since recognition was necessary to ensure Mexican Neutrality in World War I.[6][7] After the military invasion of Veracruz in 1914, Mexico would not participate in any military excursions with the United States in World War I,[8] thus ensuring Mexican neutrality was the best outcome that the United States could hope for, even if Mexican neutrality would allow German companies to keep their operations in Mexico open.[9]