Deep in the president's budget released Monday—in Table S-8 on page 161—appear a set of proposals headed "Reform U.S. International Tax System." If these proposals are enacted, U.S.-based multinational firms will face $122.2 billion in tax increases over the next decade. This is a natural follow-up to President Obama's sweeping plan announced last May entitled "Leveling the Playing Field: Curbing Tax Havens and Removing Tax Incentives for Shifting Jobs Overseas."

The fundamental assumption behind these...