One of the key lines of investigations after the 9/11 New York attack on the World Trade Centre was to follow the money. It was a complex path that led from well-known US and international banks, through a series of "correspondent", friendly, local banks in the Middle East, and back to Saudi Arabia.
Until 9/11, US money-laundering laws largely had been used by the FBI and other authorities to intercept crime and drug money. But in an age of al-Qaeda and small cells of terrorists spread across the world, cutting off access to finance became a priority.
Such cash flows have been a target of a series of probes by Senator Carl Levin's Sub-committee on Investigations, one of Congress's key battering rams. Bank regulators have been armed with financial sanctions legislation aimed notably at Iran and terrorist organisations that include the military wing of Hizbollah, as well as Hamas.
Big US banks decided long ago that it would unpatriotic and dangerous to engage in terrorist-related activities.
Overseas banks operating in the US felt few such reservations. London-based HSBC and Standard Chartered were discovered to have engaged in both money laundering and sanctions busting and were heavily fined. HSBC had a close relationship with a Saudi institution with a history of funnelling funds to terrorist groups and was fined $1.9 bn in 2012.
Standard Chartered processed dollars on behalf of Iranian institutions, undermining efforts by the West to isolate Tehran.
The ability of Isis fighters to rout Iraqi troops in Mosul and advance towards Baghdad looks strategically surprising. After all Isis - jihadis loosely connected to al-Qaeda - were largely halted by Assad in Syria. In fact, the flow of funds from oil-rich Gulf states has been unrelenting as they seek to check Shia expansionism across the region.
Isis has been empowered by the war against Damascus. Not only has it been enriched by Sunni money, it has also extracted funds from Western governments who handed over millions of dollars for the release of aid workers and other citizens during their reign of terror along the Turkish-Syrian border.
Terrorist groups do not necessarily need bank accounts to pursue their ends. When Mosul fell to Isis, it was not just Western helicopters and weapons systems that were grabbed. The vaults of the banks were also cleared of an estimated $500m. This may be the worst moment for the West to take its foot off the throat of the banks.