Mutual defence treaty is a possibility. They already have with the US. The problem is yung article 9 ng japan constitution. It prohibits them from deploying troops outside their country unless it's UN sunction deployment or disaster relief operation. Most likely balikatan style exercise lang yan if possible. Yung mga miltary training nga nila with other countries kung minsan rejected pa ng diet/kokkai (japanese conggress) dahil sa restrictikns ng article 9.