Complex sentence: The primary mechanism for enforcing the compliance of foreign financial institutions is a punitive withholding levy on U.S. assets which the Economist speculated in 2011 might create an incentive for foreign financial institutions to divest or not invest in U.S. assets, resulting in capital flight.
Split into simpler sentences: The primary mechanism for enforcing the compliance of foreign financial institutions is a punitive withholding levy on U.S. assets. This may create a strong incentive for foreign financial institutions to divest (or not invest) in U.S. assets, resulting in capital flight.