Answer:
The Articles of Confederation created a Nation that was "a league of friendship and perpetual union,” but it was the state governments that had most of the power under the Articles, with little power given to the central government.
The railroad opened the way for the settlement of the West, provided new economic opportunities, stimulated the development of town and communities, and generally tied the country together.
The Monroe Doctrine was impossible for the US to enforce, as despite the threat, they had no power to watch over and govern foreign nations. It was reliant on other nations overestimating the power of the US, and that fear was the only thing that made it of any use.
Citizens participated in the government of Athens by voting directly on issues that affected the state in the Assembly. This was a form of "direct democracy" as opposed to "representative democracy", which exists in the United States.