To Defeat ISIS Break with Saudi Arabia
ISIS rules an area the size of Britain and exports millions of dollars worth of oil so no simple measure will defeat it, but rather than endless bombing of Syrian and Iraqi cities, one very peaceful but powerful action the "West" could take would be to break with extremists regimes like Saudi Arabia once and for all.
ISIS ideas are not very different than the hatred of other religions and sects favored by the Saudi government. It's conquests and cruel treatment of captives is similar in many ways to how the Saudis conquered most of Arabia as they set up their kingdom.
The degree to which Saudis and other Gulf billionaires actually help ISIS materially can be debated, but it would send a powerful signal if the West stopped pampering religious extremists in the Arabian Peninsula and the Gulf. Stop selling them weapons, stop supporting their aggressions, stop taking their money.