What significant action did Abraham Lincoln take during his presidency?

Abraham Lincoln is best known for his significant action of freeing the slaves in the United States through the issuance of the Emancipation Proclamation in 1863. This executive order declared that all enslaved people in the Confederate states were to be set free. Lincoln's decision was pivotal in changing the legal status of millions of enslaved individuals and was a crucial step towards ending slavery in the U.S.

This action not only aimed to weaken the Confederacy during the Civil War but also laid the groundwork for the Thirteenth Amendment, which permanently abolished slavery in the United States. Lincoln's commitment to ending slavery was fundamental to his presidency and remains one of his most enduring legacies in American history.

The other options represent different historical figures or contexts: the Declaration of Independence was signed in 1776, well before Lincoln's time; the New Deal was initiated by Franklin D. Roosevelt in response to the Great Depression; and World War I involved U.S. leadership under President Woodrow Wilson, not Lincoln. These historical details help differentiate Lincoln's actions from those of other pivotal figures in U.S. history.