^^Clearly if rebels finance their activitys through smuggling and drug running as the Taliban do in Afghanastan, reducing that flow of income has multiple effects.
Rebels often thrive on smuggling goods bound for Western markets. A reduction in heroin prices in Europe for example, would have directly reduced the smugglers revenue earned by the Taliban as they rose to power in the mid 1990's.
The same is true of rebels in Afghanastan today and possibly also in the Federally Administered Tribal Areas (FATA) of Pakistan, where opium profits often make rebel forces better armed and better paid than government troops.
The revolutionary Armed forces Of Colombia (FARC) is another terrorist organiseation that finances itself with revenues from the drug trade, in this case from cocaine. (The Bekka valley east of Beirut, where Hezzbollah was initially established, was a major opium production and distribution center during the Lebanese civil war, under the protection of Syria, though it's unclear if Hezzbolla gained revenue from production). Legalizing drug use-that is, treating illicit drugs as controlled substances that are legal to grow and regulated by the government- has other social costs. Yet it might be worth considering as a way to stabilize Afghanistan, Columbia and perhaps western Pakistan, where the government is outgunned and outbribed by wealthy narcotics traders.
Petroleum is legal, of course, but by the same token, curbing global demand for petroleum would not only have environmental benefits.
It would also reduce funds available for donations to Islamist terrorism and to Islamist social services provided by rebel organisations that compete with governments, not only in the Middle East, but in many other parts of the Muslim world as well.

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CHILCUTT~
The secret to a long life. Is knowing when it is time to leave.