Waiting for Japan to fire preserves political legitimacy

Would the United States be willing to strike Kido Butai first after discovering it?

Possibly, but waiting for Japan to attack Oahu before launching the prepared ambush would give Roosevelt a much stronger political position.

An informed United States could attack before Japan formally opened hostilities, and Roosevelt might accept that risk. Doing so would create a lasting argument over legal authority and political legitimacy. A cleaner course is to prepare the carriers and defenders, then wait until the first Japanese wave attacks Oahu. American aircraft could launch as that wave returned, using its approach as cover and catching Japanese decks at a vulnerable point. Prior knowledge therefore changes readiness without automatically settling who fires first.