Complex sentence: The attack failed, however the fact that Confederate troops advanced to the outskirts of Washington D.C. caused widespread panic and prompted Lincoln to issue a call for 500,000 troops to serve a brief term of 100 days to bolster defenses around the capital.
Split into simpler sentences: The attack failed, however the fact that Confederate troops advanced to the outskirts of Washington D.C. caused widespread panic. This prompted Lincoln to issue a call for 500,000 troops to serve a brief term of 100 days to bolster defenses around the capital.