Which significant action did Abraham Lincoln take during his presidency?

Abraham Lincoln's most significant action during his presidency was the Emancipation Proclamation, which declared that all enslaved people in the Confederate states were to be set free. This landmark decision, issued on January 1, 1863, not only marked a pivotal shift in the Civil War by transforming the fight against secession into a battle for freedom but also laid the groundwork for the eventual abolition of slavery throughout the United States. The Emancipation Proclamation changed the character of the war and allowed for the enlistment of African American soldiers in the Union Army, further underscoring its importance.

The other options, while significant in their own contexts, do not pertain to Lincoln’s presidency. The expansion of the economy refers to broader economic policies that occurred later, the New Deal was initiated by Franklin D. Roosevelt in response to the Great Depression, and the Revolutionary War was fought long before Lincoln's time, making them unrelated to his actions and legacy as president.