The term “oil crisis” is used to describe two periods during the 1970s when the oil supply on the world market decreased as a result of political crises, leading not only to higher oil prices but also to an economic crash. The first oil crisis was triggered by the 20-day Yom Kippur War between Israel on one side and Egypt and Syria on the other. The Organization of the Petroleum Exporting Countries (OPEC), dominated by Arab countries, cut back on production in order to pressurize western countries into withdrawing support for Israel. The second oil crisis followed in 1979–1980, after the monarchy in Iran under Shah Mohammad Reza Pahlavi ended with the Islamic Revolution. Soon afterwards, Iraq declared war on Iran. The political instability caused another shortage of oil on the world market. The oil crises ultimately led many countries to tap their own new reserves in order to become more independent; one example is oil production in the North Sea.