Ayatollah Ali Khamenei
Khameni singled out the U.S., Britain and Israel as "big enemies."
The nuclear deal went into effect this weekend.
Ayatollah Ali Khamenei said the U.S. aids instability in the Middle East.
Cooperation was dependent on Iran agreeing to nuclear deal, the letter said.
God "brought the Americans into our waters so they would be arrested," the ayatollah says.
The agreement will insure Obama's legacy, but it will not bring peace to the Middle East.
He said Iran's willingness to prove it isn't developing a nuclear weapon will determine the deal.
Ali Khameni said Iran should not wait for sanctions to be lifted.