The US Constitution is what set the United States up for oligarchic expansionism / imperialism. It replaced the Articles of Confederation (against stiff opposition and dissent, cf.

the Antifederalists and Shay's Rebellion) which set very strict & strong limits on the powers of the federal government (and which had a president who was just the presiding officer, the actual origin of the word)

tl;dr the (controversial) adoption of the US Constitution is when the US went from decentralised to centralised