>>>What former President used the military to save the republic and what occurred exactly?
What It Takes to Save the Republic: Lincoln as Commander in Chief
"Based on these powers, he called out the militia, authorized increases in the size of the regular army and navy, expended funds for military purchases, deployed military forces, blockaded Southern ports, suspended the writ of habeas corpus in certain areas, authorized arbitrary arrests, and empaneled military tribunals to try civilians in occupied or contested areas. Later he authorized conscription and issued the Emancipation Proclamation. Lincoln justified these steps as necessary to save the Union and preserve the Constitution."
http://ashbrook.org/publications/onprin-feb2009-owens/