The British were quite justified in withholding the Zimmerman Telegram from the United States. This move allowed the British to truly get what they needed for the war. If the British had not done shown it at the time they did, then the war might still be going on.
With total desperation, the British had begged and pleaded with the United States to enter the war. Until February 24, 1917, the United States had tried to remain
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that Mexico might have made would have been entirely out of the blue and we might not have been able to prevent further damage.
The British made their point with the time of informing. The United States acted on it and therefore made the turning point in the war. If not for the cryptographers of Great Britain and the planning of their government, history would be changed entirely. But then again, history was changed entirely.