Atlantic Theater

German discovered that America kept sending aids to Allied Powers, so they sent the Zimmermann Telegram to Mexico, demanding a surprise attack against United States, in return they got their lost lands in Mexican-American War. The telegram was sent out one day delayed, since it was arguable the thing that may destroy Germany. If it was discovered by the Allied Powers, Mexico of course wouldn't help them while fresh American army would keep arriving. However, it was still sent out.

Mexican Surprise Attack
See Battle of Texas. Their attack was stopped at mid-Texas. Trench warfare started.

California Offensive
MacArthur, leading U.S. 42nd Division, broke through Mexican defense in California easily. They kept pushing in West, until they were lack of resources and returned to Lower California. They had reached Mexico City once, but due to the lack of seige tools, they retreated.

Atlantic Theater
United States of America sent many troops to Germany. However, because of the bad weather, some transportation ships was lost and disconnected with American Navy. Also, German U-boat, after improvement after getting some naval skills after capturing Paris, sank many American transportation ships. It created fear to the Americans, and American's effort to send oversea troops decreased a lot. The Germans also could launch a naval defense in France since coastal cities was almost all captured. Moreover, Battle of Texas took place, so the United States decided to defeat Mexico first. However, due to trench warfare in mid-Texas, American couldn't be sent to Europe but to Texas against Mexican trenches.