Carnegie believed that the wealthy had an obligation to share their wealth to improve society and help others.
Carnegie wrote his beliefs about wealth in the <em>Gospel of Wealth </em>written in 1889.
Carnegie practiced what he preached. He shared his wealth with many organizations in particular libraries, universities, and artistic institutes. He wealth was used to support the educational community of the US and provide opportunities for others to benefit from his wealth. Many other wealthy members of society have followed in his model by using their wealth to make the US a better place.
The civilization of Olmec.
This is a bit subjective, but one could argue that the Fourteenth Amendment, which was one of the Reconstruction amendments, had the biggest impact on American history since it declared that all American citizens must have equal rights and protections under the law.