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Now what if The German government gains a few IQ points and decides not to send the Zimmermann note. However at the same time what if the British Government attempts to bring the US into the war by forging a note or through other means. I've heard some things about how Churchill plotted just such a thing during both world wars though I know nothing of how accurate such claims are.
But basically the British government tries to trick the US into entering the war and is caught.