What was one important thing Abraham Lincoln did?

The USCIS Civics Question 72 asks for one important thing Abraham Lincoln did. Three answers are acceptable: FREED THE SLAVES — Lincoln issued the Emancipation Proclamation on January 1, 1863, declaring all enslaved people in the Confederate states to be free; the 13th Amendment (1865) then abolished slavery throughout the entire nation; SAVED/PRESERVED THE UNION — Lincoln led the country through the Civil War (1861-1865) and preserved the United States as one nation; the Southern states had seceded and formed the Confederate States of America; LED THE UNITED STATES DURING THE CIVIL WAR — Lincoln was the 16th President (1861-1865) during the bloodiest conflict in American history. Lincoln was born in a log cabin in Kentucky in 1809; he taught himself law; served in the Illinois state legislature; became President in 1861; was assassinated on April 14, 1865, just days after the Civil War ended.