Complex sentence: In January 1917 Germany made a major strategic blunder that perhaps cost it victory in the war Germany launched a full-scale blockade of Britain, using its U-boats to sink without warning in violation of international law and its solemn promises to the United States.
Split into simpler sentences: In January 1917 Germany made a major strategic blunder that perhaps cost it victory in the war. This was in violation of international law and of its solemn promises to the United States.