Originally Posted by
CPH-Flyer
Japan has no requirements for transit. But as you will be arriving on an international flight to Guam, you would need to fulfil the entry criteria for entry in to the US. There is no test on arrival, but a pre departure test will be required.
That is not entirely true.
https://www.cdc.gov/coronavirus/2019...travelers.html
If you booked an itinerary from a U.S. state or territory to another U.S. state or territory and the itinerary has you taking a connecting flight through a foreign country, CDC does not require that you be tested. An example of this situation is an itinerary booked between the Northern Mariana Islands (a U.S. territory) and the U.S. mainland via Japan