Verified answer

We officially became a Corporate Oligarchy when we allowed Corporations to make campaign contributions (i.e. legalized bribes) to politicians as if they were people.

We Unofficially became a corporate Oligarchy when we allowed campaign contributions (i.e. legalized bribes) in the first place.

Probably shortly after it was changed into a "Democracy"

"A democracy cannot exist as a permanent form of government. It can only exist until the voters discover that they can vote themselves largesse from the public treasury." -- Alexander Fraser Tyler, in 1776 book, "The Decline and Fall of the Athenian Republic,"

“The general object was to produce a cure for the evils under which the United States labored; that in tracing these evils to their origins, every man had found it in the turbulence and follies of democracy.” -- Edmund Randolph of Virginia, describing the effort to deal with the issue at the Constitutional Convention

"Remember, democracy never lasts long. It soon wastes, exhausts and murders itself. There never was a democracy yet that did not commit suicide." -- John Adams

US constitution Art 2 Section. 4. The United States shall guarantee to every State in this Union a Republican Form of Government, and shall protect each of them against Invasion;......

You mean an Obama socialist cronyocracy?