For decades, global finance firms eagerly catered to Russian firms, billionaires and the government. Then tanks started rolling into Ukraine.

Citigroup Inc., which has thousands of staff and billions of dollars of assets in Russia, has said it will cut back much of its business in the country. Goldman Sachs Group Inc., JPMorgan Chase & Co. and Deutsche Bank AG are also heading for the exit, with some financiers relocating to other hubs such as Dubai. They’re being followed by lawyers and other professionals.

It’s perhaps the harshest and fastest exclusion in living memory of a major industrialized economy. The past few weeks have been a frantic dash to understand and implement sanctions that are being continually updated by jurisdictions including the U.S., U.K., Japan and the European Union.