Crude gushing into the Gulf of Mexico and washing ashore in Louisiana is exposing how ill-prepared the U.S. has been to respond to a major offshore oil spill.

In the fight to limit environmental damage from the month-old spill—which is on track to rival the 1989 Exxon Valdez disaster in size—BP PLC executives, government officials, and scientists are learning as they go, even though the industry has been drilling in the Gulf for decades and has 77 rigs operating there, according to ODS-Petrodata, a research firm.

The...