This paper studies the long-term economic cycles of the international price of the oil barrel written in the supply contracts signed after January 1972. The approach begins by the estimation of the long-term trend using the Hodrick-Prescott filter, and continues with the isolation of the long-term cycles that can be identified from the data series. In a second step it dissects each one of the long-term cycles in the different expansion and contraction periods of the price cycles. In the last step it appreciates the existence of an association among these cycles and their phases, the armed conflicts that occurred during these 30 years, the main political leaders of the countries that played the main role in the international scene (some countries of the EU, the USA and Russia) and other important explicative factors of the long-term evolution of the international crude oil price.