American ships in a Mers-el-Kébir counterfactual

What if Britain accidentally sank American battleships and carriers at Mers-el-Kébir?

Warnings and an apology would be expected. A war would be dangerous for Britain, but the premise requires major changes to American conduct and the circumstances of the attack.

British forces would be expected to warn American battleships or carriers present at Mers-el-Kébir and share the options offered to the French admiral. An accidental sinking would initially bring an apology. The scenario must explain the Americans' presence, their decision to remain after warnings and positions close enough to French ships for confusion. The British negotiator was their most senior available French-speaking officer and had previously served as naval attaché in Paris, yet his seniority was still judged insufficient. Normally, American and French ships would occupy distinct areas of the harbour. An American declaration of war would create another front for Britain, including Canada. It would also align America with the fascist powers, contrary to its diplomatic and military direction at that point.