Researchers have for the first time provided strong evidence for what conspiracy theorists have long thought -- oil is often the reason for interfering in another country's war.

Throughout recent history, countries which need oil have found reasons to interfere in countries with a good supply of it and, the researchers argue, this could help explain the US interest in ISIS in northern Iraq.

Researchers from the Universities of Portsmouth, Warwick and Essex modelled the decision-making process of third-party countries in interfering in civil wars and examined their economic motives.

They found that the decision to interfere was dominated by the interveners' need for oil over and above historical, geographical or ethnic ties.

Civil wars have made up more than 90 per cent of all armed conflicts since World War II and the research builds on a near-exhaustive sample of 69 countries which had a civil war between 1945 and 1999. About two thirds of civil wars during the period saw third party intervention either by another country or outside organisation.

Dr Petros Sekeris, from the University Portsmouth, Dr Vincenzo Bove, from the University of Warwick, and Dr Kristian Skrede Gleditsch from the University of Essex, wanted to find out which factors made it more likely that a third party state would militarily intervene in an ongoing intrastate war.

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Dr Sekeris said: "We found clear evidence that countries with potential for oil production are more likely to be targeted by foreign intervention if civil wars erupt.

"Military intervention is expensive and risky. No country joins another country's civil war without balancing the cost against their own strategic interests and what possible benefits there are.

"We wanted to go beyond conspiracy theories and conduct a careful, nuanced analysis to see whether oil acts as an economic incentive in the decision on whether to intervene in an internal war in another country.

"The results show that outsiders are much more motivated to join a fight if they have a vested financial interest."

Among the findings, published in the Journal of Conflict Resolution, are:

The more oil a country has, the more likely a third party will intervene in their civil war;

The more oil a country imports, the greater the likelihood it will intervene in an oil-producing country's civil war;

Dr Bove said: "Before the ISIS forces approached the oil-rich Kurdish north of Iraq, ISIS was barely mentioned in the news. But once ISIS got near oil fields, the siege of Kobani in Syria became a headline and the US sent drones to strike ISIS targets.

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"We don't claim that our findings can be applied to every decision made on whether to intervene in another country's war, but the results clearly demonstrate supply of and demand for oil motivates a significant number of decisions taken to intervene in civil wars in the post-World War II period.

"The 'thirst for oil' is often put forward as a near self-evident explanation behind the intervention in Libya and the absence of intervention in Syria. Many claims are often simplistic but, after a rigorous and systematic analysis, we found that the role of economic incentives emerges as a key factor in intervention."

The research found that a third party country was more likely to intervene if:

They were a major power;

The rebels were strong and well-armed;

There were close ethnic ties between the two countries; and/or

The civil war took place during the Cold War, a period of global competition between superpowers.

Among the examples highlighted by the researchers were USA's involvement in Angola's civil war from 1975 to the end of the Cold War and in Guatemala, Indonesia and the Philippines, and the USA's support of conservative autocratic states in oil-rich. Also cited were the UK's involvement in Nigeria's 1967-70 civil war, in contrast to the non-intervention in civil wars in other former colonies which had no oil reserves (Sierra Leone and Rhodesia, later Zimbabwe); and the former Soviet Union's involvement in Indonesia (1958), Nigeria (1967-68) and Iraq (1973).

The researchers said that at the other end of the spectrum, oil-rich states including the Gulf States, Mexico and Indonesia have no history of military intervention in other countries' civil wars, even if they have advanced and well equipped military forces. With the West becoming less energy-dependent and China becoming more energy-dependent, the incentives for third party countries to intervene in other countries' wars was likely to change in the future, they said.