In 1917.
They didn't. Mexico remained neutral during WW1. The Germans tried to enlist their support by offering then Texas, New Mexico, and Arizona. The Mexican government turned down the offer and stayed out of the war.
United States, Mexico, Germany The Zimmerman Telegram (1917) was sent by the foreign minister of Germany to the German consul in Mexico, suggesting that Germany would support Mexico in a war with the US.
Germany offered Mexico the military and financial support needed to retake Texas, New Mexico and Arizona; territories lost during the Mexican-American War (1846-1848).
They German foreign minister is prepared to offer Mexico financial support if they go to war with the United States.
Germany offered Mexico financial and military support to take back New Mexico, Texas and Arizona from the United States.
An intercepted messege from Germany to Mexico, telling them that if they join sides with Germany, and attack the US; when the war's over, Germany will ensure that the defeated US returns the lands that they took from Mexico during the Mexican War of 1846-1848. Those lands are called California, New Mexico, Arizona, and Nevada today. Mexico firmly believed that Germany couldn't win the war (WWI), and consequently declined the offer.
In the Zimmermann Telegram, Germany tried to convince Mexico to start a war with the US, which would prevent the US from joining the Allies during World War I. In return, the Germans promised to send Mexico money and supplies to help fight their war. Additionally, Germany promised that if they won the war, Mexico would be allowed to take back Texas and California, which the US had taken from Mexico about 60 years earlier.
texas, California, Nevada
There were several contributing factors. One was the sinking of the ocean liner Lusitania by German submarines, and the deaths of Americans on board the ship. The other was the "Zimmerman Telegram", where Germany sent a message to Mexico seeking to have Mexico attack the US. Mexico would receive financial support from Germany, and be given Texas, Arizona, and New Mexico.
Mexico did, but indirectly. In the middle of WWI, Germany sent a telegram to the Mexican president, known as the Zimmerman Telegram. In it, Germany offered financial and military support to Mexico to reclaim Texas, New Mexico and Arizona from the United States. Before Mexico could answer the telegram, it had already been intercepted by British operatives in Mexico City, and retransmitted to Washington. It enraged president Woodrow Wilson, who used the telegram to support the cause for entering WWI. It did.
Parts of the US they lost in Mexican-American war 1848
Mexico and Germany