Ratification of the 14th Amendment.
During the Civil War there was a tension between the federal government and some states regarding the institution of slavery and whether the federal government had autonomy to genuinely abolish or regulate this matter. The Emancipation Proclamation was a law issued by President Abraham Lincoln that declared free all the people who were held as slaves in the rebellious states and led to the acceptance of the 13th Amendment that abolished slavery in the United States once and for all. However, the best evidence that the Civil War greatly increased the power of the federal government was the ratification of the 14th amendment which guaranteed citizenship to all individuals either born or naturalized in the U.S - including former slaves.