What was one important thing that Abraham Lincoln did?

One important thing that Abraham Lincoln did was to preserve the Union during the Civil War. Lincoln's leadership was crucial in navigating the nation through its most challenging conflict, which not only involved military strategies but also required profound political decisions and moral clarity. He recognized that the preservation of the Union was essential not just for the survival of the United States but also for the principles of democracy and freedom that it represented. Lincoln took bold actions such as issuing the Emancipation Proclamation, which shifted the purpose of the war towards ending slavery, and he consistently worked to maintain the loyalty of the border states and to manage tensions within both the North and the South. His efforts culminated in the eventual victory of the Union, ensuring that the United States would remain a single nation.

The other options, while related to the era, do not accurately reflect Lincoln's direct contributions. Lincoln did not personally fight in the Civil War, although he oversaw the military efforts. The Treaty of Paris is associated with the end of the American Revolutionary War, not Lincoln’s presidency, and the establishment of National Parks occurred much later, under different leadership.