The Zimmerman Telegram was a telegram from Germany to Mexico containing crucial war information about The Great War. It included the Germans' plans for unrestricted submarine warfare, as well as a proposal asking Mexico to ally with the Germans and invade the US. The Germans had hoped to attack the US on three fronts: Mexico from the South, Japan from the West, and Germany from the East.
However, this telegram was intercepted and decrypted by the British, led by Admiral Sir William Hall. Upon reading the telegram, rather than warning the Americans about the U-Boat warfare that was about to ensue, he decided to keep the telegram a secret. He did this because he knew that if America publicly condemned Germany's acts of aggression, the Germans would know that their encryption system had been compromised, and strengthen it. Admiral Hall was thinking long-term; he knew that the Germans could not win the war.
While Admiral Hall's decision may not have been completely ethical, I do believe it was the right decision to make. Yes, American lives were compromised due to the unrestricted submarine warfare, but if the Germans had changed their encryption the Allies may not have won the war. In wartime, it's imperative that the most strategic decisions are made. In eventually using the stolen telegram from Mexico to convince America to enter the war, I believe Hall found a happy medium between handing over the telegram and keeping it a secret. Many times, secrets are necessary if kept for the greater good.