natural resources that they could smuggle. A 2004 literature review finds that oil makes the onset of war more likely and that lootable resources lengthen existing conflicts. One study finds the mere discovery (as opposed to just the exploitation) of petroleum resources increases the risk of conflict, as oil revenues have the potential to alter the balance of power between regimes and their opponents, rendering bargains in the present obsolete in the future. One study suggests that the rise in mineral prices over the period 1997–2010 contributed to up to 21 percent of the average country-level violence in Africa. Research shows that declining oil prices make oil-rich states less bellicose. Jeff Colgan observed that oil-rich states have a propensity to instigate international conflicts as well as to be the targets of them, which he referred to as "petro-aggression". Arguable examples include Iraq's invasions of Iran and Kuwait; Libya's repeated incursions into Chad in the 1970s and 1980s; Iran's long-standing suspicion of Western powers; the United States' relations with Iraq and Iran. It is not clear whether the pattern of petro-aggression found in oil-rich countries also applies to other natural resources besides oil. Some scholars argue that the relationship between oil and interstate war is primarily driven by the case of the Iran-Iraq War and that the overall evidence points in the direction of an oil-peace. A 2016 study finds that "oil production, oil reserves, oil dependence, and oil exports are associated with a higher risk of initiating conflict while countries enjoying large oil reserves are more frequently the target of military actions." As of 2016, the only six countries whose reported military expenditures exceeded 6 percent of GDP were significant oil producers: Oman, South Sudan, Saudi Arabia, Iraq, Libya, Algeria. (Data for Syria and North Korea were unavailable.) A 2017 study in the American Economic Review found that mining extraction contributed to conflicts in Africa at the local level over the period 1997–2010. A 2017 study in Security Studies found that while there is a statistical relationship between oil wealth and ethnic war, the use of qualitative methods reveals "that oil has rarely been a deep cause of ethnic war." The emergence of the Sicilian Mafia has been attributed to the resource curse. Early Mafia activity is strongly linked to Sicilian municipalities abundant in sulphur, Sicily's most valuable export commodity. A 2017 study in the Journal of Economic History also