South Korea and the United States on Aug. 19 began regular annual military exercises, including operations geared toward North Korea, but this year there is a difference. Japan is a more visible defense partner in the Pacific and beyond.

This follows directly from a Camp David summit last year hosted by President Joe Biden that included the prime ministers of both Japan and South Korea. The resulting agreement greatly changes the dynamics of the balance of power in Northeast Asia, and more generally in the vast continent of Asia.

Traditionally, Japan and Korea were enemies. The powerful military culture of historically unified Korea generally maintained independence against China and Japan. This changed in 1905, when Japan invaded and occupied Korea.

That occupation continued until Japan’s defeat in 1945. Soviet forces occupied Korea north of the 38th Parallel, U.S. forces controlled the southern region. As with Germany, the occupation zones were conceived as a temporary expedient among the victorious Allies, but the onset of the Cold War made them permanent. Germany is now united, but Korea remains divided.

So far, both China and North Korea have been relatively muted in publicly complaining about these exercises. That may reflect an intimidation factor given Japan’s increasing prominence in defense..

Additionally, both communist nations along with Russia are beleaguered. Russia’s President Vladimir Putin has moved to strengthen military ties with Pyongyang, but that reflects the unanticipated great costs resulting from Moscow’s invasion of Ukraine.

China’s economy is in growing difficulty. Beijing strives to attract private investment while simultaneously maintaining a brutal police state. In effect, the regime is trying to ride two horses going in opposite directions.

Beijing’s military assertiveness, especially at sea, is one important incentive for the expanding military role of Japan. In 2016, a panel of five judges at the Permanent Court of Arbitration in The Hague ruled unanimously against China and in favor of the Philippines regarding territorial disputes. Philippine-U.S. military cooperation is expanding, while China tensions continue.

There are several important regional initiatives underway in addition to the reinforced Japan-South Korea-United States partnership. Among others, the Quadrilateral Security Dialogue (known as the Quad) involves Australia, India, Japan and the U.S.

Erratic North Korea behavior is now routine. In 2013, the regime announced a “state of war” with South Korea and threatened nuclear attack. Pyongyang abruptly abrogated the 1953 armistice agreement ending the Korean War, and cut the military “hot line” communications link with the south.

In March 2010, a North Korean torpedo sank the South Korean ship Cheonan. In the same vicinity in November of that year, North Korean artillery bombarded South Korea’s Yeonpyeong Island.

North Korea has conducted rudimentary nuclear weapons tests along with missile tests. Progress has been uneven, but there has been slow expansion of missile ranges. There is now evidence of capacity to strike the continental U.S., and the ominous effort seriously threatens Japan as well as South Korea.

In 2017, the United Nations Security Council passed Resolution 2271 in response to North Korea intercontinental ballistic missile tests. This motion involving severe economic sanctions passed unanimously, but since then both China and Russia have moved to support North Korea..

In 2016, the U.S. and South Korea agreed to deployment of the Terminal High Altitude Area Defense anti-ballistic missile system given Pyongyang’s missile program. This is a defensive system, therefore stabilizing.

This introduces the most fundamental point. For all the tensions and strains along the 38th Parallel, and occasional violent incidents, the 1953 armistice has held. Presidents Harry Truman and Dwight Eisenhower deserve credit for deciding to defend South Korea and then ending the war.

Arthur I. Cyr is author of “After the Cold War — American Foreign Policy, Europe and Asia.”