Purple was the code name Americans gave an Imperial Japanese Navy diplomatic cipher that was used to send messages to important diplomatic offices around the world. There's no question that in the lead up to World War II the Americans became increasingly interested in what the Japanese were communicating over Purple.
The Purple machine was first used by Japan in 1939. But US and British cryptographers had already broken some of its messages well before America's entry into the war. The intelligence they gleaned from Purple was code named Magic.
One of the most important pieces of Magic US cryptographers "found" was the 14-part Japanese diplomatic message ominously breaking off relations with the United States. That was at 1PM Washington time, December 7, 1941.
The message was a sure sign that a Japanese attack against America was imminent, but nobody knew where. The Japanese were careful not to communicate any of their war plans to the Japanese Foreign Office as it was considered insufficiently reliable by military leadership.
A few hours later the whole world would know the answer, anyway. Pearl Harbor was the destination, and more than 2,000 Americans would lose their lives.
As part of the museum's permanent collection, there are two, 1940s-era Japanese Purple machines one of which was the actual machine that received the 14-part diplomatic message breaking off relations with the United States hours before the attack on Pearl Harbor.