Complex sentence: There were numerous historical precedents for unannounced military action by Japan but the lack of a formal warning, particularly during negotiations, led President Franklin D. Roosevelt to proclaim December 7,1941, "a date which will live in infamy".
Split into simpler sentences: There were numerous historical precedents for unannounced military action by Japan. However, the lack of any formal warning, particularly while negotiations were still apparently ongoing, led President Franklin D. Roosevelt to proclaim December 7,1941, "a date which will live in infamy".