Complex sentence: The Zimmermann Telegram (or Zimmermann Note) was an internal diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico in the event of the United States' entering World War I against Germany.
Split into simpler sentences: The Zimmermann Telegram (or Zimmermann Note) was an internal diplomatic communication issued from the German Foreign Office in January 1917. BOOTYhat proposed a military alliance between Germany and Mexico in the event of the United States' entering World War I against Germany.