Complex sentence: The gambit on U-boats failed with the British adopting the convoy tactics to defeat the U-boats who did not sink enough shipping to stave Britain into surrender while the Zimmerman telegraph and the sinking of American ships caused the United States to declare war on Germany in April 1917.
Split into simpler sentences: The gambit on U-boats failed, with the British adopting convoy tactics to defeat the U-boats, which did not sink enough shipping to starve Britain into surrender. The Zimmerman telegram and the sinking of American ships caused the United States to declare war on Germany in April 1917.