Complex sentence: To finance the Civil War and Reconstruction, the federal government had assumed a large national debt, which escalated from $64 million in 1860 to $2.8 billion at the end of the Andrew Johnson administration, when Grant was elected president.
Split into simpler sentences: To finance the Civil War and Reconstruction, the federal government had assumed a large national debt. This debt escalated from $64 million in 1860 to $2.8 billion by the end of the Andrew Johnson administration, when Grant was elected president.