Would the US declare war first if it saw a serious threat to the British empire after a Japanese invasion?
Its not difficult to get the US into this Japa/European War. OTL the US arranged to sell large quantities of arms to Germanys enemies & restricted sales of anything to Germany by requiring cash, imposing buercratic delays, and other obstacles. The US arraigned a Neutrality or Exclusion Zone for a extended distance from its territory. That Exclusion zone was extended & eventually was expressed in a US Naval base and Marines inside the war zone on Iceland. The US provided its military personnel as trainers where the French and British purchased its weapons. Germany not wanting to provoke the US went along, but by the spring of 1941 was starting a clandestine war between agents in Latin American, in the autumn a naval war in the Atlantic.
Previous to Japans attack in December 1941 the Dutch, US, and Britain had been systematically sharing intelligence among their service in the Pacific and Asia. They had also been consulting at the highest levels for joint war plans. So its not like the US is on the other side of some sort of wall of neutrality. There is also the matter of the War Warning sent 27 November. That specified any Japanese forces sighted were to be considered "hostile". In the military parlance I was trained in that was permission to shoot.
The first thing that occurs from the first day is the US is running its usual military patrols in the region of the Philippines, Guam, Wake, ect... Those would be stepped up imeadiatly as common prudence. The US has no worthwhile assurance it will not be attacked soon. So increased patrols, and alert of the military forces is certain. The second major action would be to establish a Exclusion or Neutrality Zone around US & Philippines territory. The obvious precedent is the previous Exclusion Zone in the Atlantic. Patroling that is certain as well. Third thing is accelerate the shipment of arms and material to both the Philippines and the Allies, the Dutch and Brits. All that makes for a volatile & tense mix. Bottom line is Congress may not be consulted by naval officers on the spot, particularly Japanese naval officers making a judgment call on attacking or not on warships or a cargo ship convoy, where there is any ambiguity.