Would Britain join a US-Japanese war after Panay?

If the United States and Japan had gone to war over the Panay incident, would Britain have joined?

Probably: Britain would join as the secondary partner so that it retained influence over the war and the peace settlement.

Yonai disciplined the officer responsible for the Panay incident, illustrating why that crisis was easier to defuse than a deliberate attack at Tsingtao. If the United States nevertheless went to war with Japan over Panay, Britain would probably join. Washington would be the injured party and therefore lead, while Britain became the secondary great-power partner. Britain would intervene for the same reason assigned to America in the Tsingtao scenario: staying out would sacrifice influence over the military outcome and peace settlement.