Among US atheist circles, the 1797 Treaty of Tripoli is an oft-quoted document. Its role in the debates about religion and government is to counter the popular idea that the US was founded as a Christian nation.
I looked up this treaty for myself to see what it said, and here's the relevant text.
As the government of the United States of America is not in any sense founded on the Christian Religion,-as it has in itself no character of enmity against the laws, religion or tranquility of [Muslims],-and as the said States never have entered into any war or act of hostility against any [Muslim] nation, it is declared by the parties that no pretext arising from religious opinions shall ever produce an interruption of the harmony existing between the two countries [the United States and Triploy].
Ratified by the United States June 10, 1797
You can look it up yourself at the Avalon Project at Yale Law School.
Obviously, this text isn't the tremendous revelation sought by people who'd like to establish that the US was not founded on religion. The treaty leaves open the possibility that the US was founded on a belief in God, with religion as a bedrock of US society (just not necessarily Christianity). Additionally, there are plenty of quotes from various Founding Fathers tying their efforts to Christianity. Still, I don't know of an official statement that the US was founded on Christianity, which is what you'd want if you were looking to contradict the Treaty of Tripoli.
Related Historical Event: In 1863, eleven Protestant denominations organized the National Reform Association. Their goal was to amend the Constitution so that it would refer to the US as a Christian nation, with the government deriving its authority not from the people but from Almighty God. The campaign failed. It wasn't until almost a century later that conservatives managed to get "God" into our pledge and national motto.