The US-Japan alliance, born from the post-World War II settlement, has evolved from a relationship where the US provided most of Japan's defense to one of increasingly equal military partnership. For decades, Japan's constitution, drafted under US occupation, renounced the use of force except for self-defense, and the Japan Self-Defense Forces were established in 1954 under that constraint. However, interpretations of self-defense have broadened over time, allowing Japan to participate in international peacekeeping and, more recently, to take on a more proactive military role. This shift accelerated in December 2022 when Japan adopted a new security strategy that defined China as its primary strategic challenge and committed to acquiring long-range missile capabilities, a clear break from its self-defense-only policy.

The US has consistently supported this evolution, viewing a stronger Japan as a key pillar of its Indo-Pacific strategy. The US maintains a significant military presence in Japan, including approximately 60,000 troops, and has been modernizing its own forces there, such as replacing older F-16 jets with advanced F-35A stealth fighters at Misawa Air Base. This deepening integration is a response to a shared perception of growing threats from China, Russia, and North Korea, which have been conducting joint military activities and expanding their own capabilities. The alliance's focus has shifted from simply deterring aggression to actively building interoperable, advanced military capabilities, a process that has now reached a new milestone with the agreement to jointly produce missiles.