In the Star Trek universe, the Prime Directive is a rule that prohibits Starfleet personnel and spacecraft from interfering in the normal development of any society. This means no Starfleet mission can involve itself in a planet's internal conflicts or supply advanced technology to it. Adopted relatively early in Starfleet history, the Prime Directive was a key part of Starfleet and Federation policy toward newly discovered civilizations, but it was also one of the most difficult to administer. In most cases, the Prime Directive applied to any civilization that had not yet developed the use of warp drive for interstellar travel. This rule was not in force in 2168, when the U.S.S. Horizon contacted planet Sigma Iotia II, resulting in disastrous cultural contamination.