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 neutral in the war. This was rapidly changed when they were informed of the contents of the telegram. When the public was made aware of the contents on March 1, 1917, the already semi-pro war citizens became full throttle. The Telegram was simply a spark for the British to act on. Finally, on April 6, 1917, Congress declared war on Germany and the country supported it.
This move alone is unarguably the most significant cryptanalysis in recorded history.
The British were completely and totally justified because this was the only way that the United States would enter the war. They had to wait until the exact, right time for the telegram to make the kind of impact that it did.
The Zimmerman Telegram offered Mexico the ability to regain territories in the United States. This would be totally crossing the line due to the fact that the United States had been remaining neutral and now Germany decided to cross the line. Thankfully, the British informed the United States and they were able to act upon it. Without this information, the attack 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...