With the sinking of The British Passenger Liner RMS ''Lusitania'', The United States would formally enter The European War (World War I). Then President Woodrow Wilson (who had kept America out The War for almost it's entirety), still supported The European Allies with War Materials. Many ''War Hawks'' rallied for entry into The War against Imperial Germany. Prior to that tragic sinking, a document came to American Intelligence light that many Historians cite was enough to bring The US into The War. This was The ''Zimmerman Telegram''.
Imperial German was well aware that if The US came into The War officially, they would lose. The US could produce more equipment and mobilize more Troops, and such an action would have reinforced a bogged down Great Britain and Her Allies. Secret discussions between Germany and Mexico (a German Ally) as early as 1914 served the following goal. If Mexico could maintain an impromptu Guerrilla War along the Southern Borders between them and The US, such pressure would divert the need to formally enter The War. While Mexico did not wish ''open War'', it was ''promised'' that not only would Germany send Military Advisors as well as give Military Aid, - should Germany and it's planned European Offensive be successful in France, they would declare War on The US (and Mexico would as well). The ultimate reward – to regain all lost Territory from The Mexican American War of the mid-1840s (or much of The American West and Southwest).
Enter the unofficial plan. Guerrilla Warfare would ''rage'' along The Mexican-American Border in The South. ''Bandit'' incursions across The Border extended into Southern Texas, New Mexico and Arizona. These ''unprovoked'' Operations caused a mass mobilization of The US Army and many State National Guards Units – all converging not only along The Border, but even ''invading'' and occupying several Mexican Border Cities and Towns. This pressure and misdirection played into German plans effectively.
Imperial German Foreign Service Official Arthur Zimmerman was tasked with sending Advisors and written directives to The Mexican Consulate. By 1917, Germany no longer outwardly feared American involvement. Committed to increasing U-Boat Operations in The North Atlantic (to counter supposed US Aid to England via ''false flag'' tactics and using previously outlawed attacks), Germany gave great discretionary powers to it's Submarine Commanders. Although denied, German Submarines could known attack any form of Shipping they deemed a potential threat.
Great Britain's Intelligence Service in 1915, intercepted a secret transmission between Attache Zimmerman and The Mexican Government, which read in part that not only would Imperial German remove all regulations (unrestricted) on it's Submarine Warfare tactics, but would support Mexico in a declared War with The United States. These statements played well into the the proof that ''Bandit'' Raids and Foreign Military Advisors were a plan declaration of War. The intercepted telegram between Germany and Mexico was relayed to The US.
Plans were discussed to fully commit US Forces to invade Mexico – yet again. However, German Submarine Commanders determined that Passenger Ships (loaded with civilians) – were actually a form of ''human shield'' for Ship Holds that really carried munitions and supplies. The ''Lusitania'' was seen as one of those ''false flag'' Vessels. In May of 1915, they would sink Her. Several hundred of Her passengers were American Women and Children. War was declared.
Oddly, The Zimmerman Telegram would have served enough to rally The US Congress to pressure President Wilson to declare War. The Mexican Incursions created much in the way of racial animosity towards Mexico, that it is speculated most Americans would have supported a second Mexican War (unaware of the German Plan of support).