Unrestricted submarine warfare clearly marks the United States shift from isolationism to interventionism. The US had several different involvements with the German who used unrestricted submarine warfare against either US citizens or the US military. For example, the Germans sunk the Lusitania, a British ship that contained American citizens. In this attack, over 100 American citizens were killed. This caused outrage among American citizens.
Tensions with Mexico is not a useful mark to show America's shift away from isolationism. The tensions between these two were only escalated due to the Zimmerman Telegram, which the Mexican government never technically received. Instead, the tensions lie more with the German government (who were responsible for sending this message).
