A U.S. investigation into the lack of write-downs at Exxon Mobil Corp. despite the slump in oil-prices has brought to light the challenge of assessing impairments under U.S. Generally Accepted Accounting Principles (GAAP). But, for international companies and U.S. firms with foreign operations, figuring out GAAP is not the only contest. For the Wall Street Journal’s CFO Journal, I have taken a closer look at write-downs under GAAP vs. IFRS.