Energy diplomacy is a form of diplomacy, and a subfield of international relations. It is closely related to its principal, foreign policy, and to overall national security, specifically energy security. Energy diplomacy began in the first half of the twentieth century and emerged as a term during the second oil crisis as a means of describing OPEC's actions. It has since mainly focused on the securitization of energy supplies, primarily fossil fuels, but also nuclear energy and increasingly sustainable energy, on a country or bloc basis.