The Diego Garcia deal bypassed legislative approval and was signed in secret

Evidence

A year later, the U.S. and British governments confirmed the arrangements with a little-noticed “Exchange of Notes.” This effectively constituted a treaty, but unlike a treaty, it required no congressional or parliamentary approval—allowing both governments to keep the plans hidden. Representatives of the two governments signed the agreement, as one of the State Department negotiators told me, “under the cover of darkness,” the day before New Year’s Eve 1966. — David Vine, Base Nation_ How U.S. Military Bases…, 175