Strategic convergence under fire: ROK-Japan cooperation is imperative

Chun In-bum

Seven Russian and two Chinese military aircraft entered South Korea’s air defense identification zone (KADIZ) on Dec. 9, remaining for roughly an hour before departing. While the incident did not constitute a violation of sovereign airspace, Seoul lodged a diplomatic protest with both Beijing and Moscow the following day. Japan also responded by scrambling aircraft to implement air defense measures against potential airspace violations.

Air defense identification zones are not territorial airspace, and international law does not prohibit foreign military aircraft from entering them. However, this incident is part of an increasingly assertive pattern in which China and Russia routinely conduct joint military flights into KADIZ without prior notification, citing exercises. These actions are deliberate signals, designed to test responses, normalize coercive behavior and exploit seams between regional actors.

This pattern reflects a deteriorating regional security environment and underscores that the Republic of Korea (ROK) and Japan face an increasingly convergent set of security challenges. These include North Korea’s advancing nuclear and missile programs, repeated Chinese incursions near Japan’s Senkaku Islands in the East China Sea, China’s unilateral deployment of aquaculture cages in the Provisional Measures Zone jointly managed with the ROK, and a series of unsafe and escalatory aerial encounters involving Chinese military aircraft near Japan’s Okinawa islands.

China is also exerting coercive pressure on Taiwan, where under the banner of Justice Mission 2025, Beijing established exclusion zones and conducted long-range live-fire drills in waters surrounding Taiwan. While it framed the exercises as a “serious warning” against “external interference,” the exercises dangerously escalated tensions in the Taiwan Strait.

Japan has consistently raised concerns about China’s behavior, positions that are increasingly shared by the ROK. In response to recent aerial incidents, partners such as Australia and the United States have publicly supported Tokyo and raised concern over Beijing’s actions. The United States, Japan, Australia and the United Kingdom also expressed their concern in response to Justice Mission 2025. These developments highlight a broader reality: The strategic pressures confronting Seoul and Tokyo are no longer parallel but overlapping.

For South Korea, rising tensions in the Taiwan Strait carry significant strategic risks. Any instability could destabilize the Korean Peninsula and disrupt vital sea lanes on which the country’s economy depends.

In the face of regional pressures, and recognizing that stability in the Taiwan Strait is increasingly linked to its own national security, deeper ROK-Japan cooperation, reinforced through close trilateral coordination with the United States, has become essential to sustaining the Free and Open Indo-Pacific strategy. Originally articulated by Japan, which is on the front line of securing this vision, its success now rests on practical cooperation rather than rhetoric alone.

On the bilateral front, diplomatic engagement between the ROK and Japan continues to expand. The two governments are preparing for a leaders’ summit in mid-January as part of a renewed commitment to maintain regular diplomacy. From Tokyo’s perspective, this approach aligns with a broader effort to deepen cooperation with like-minded partners that share democratic values and a commitment to a rules-based international order.

Importantly, President Lee Jae Myung’s agreement to pursue a “two-track approach” toward Japan — separating future-oriented cooperation from unresolved historical and territorial issues — has created critical political space for progress in security coordination. This framework reflects a growing recognition in both capitals that defense and deterrence cannot be held hostage to historical disputes.

Recent defense-focused engagements further demonstrate the deepening operational cooperation between the ROK and Japan. The Freedom Edge 2025 exercise, involving the United States alongside its two key Asian allies, focused on refining ballistic missile defense and improving trilateral interoperability. These drills coincided with major U.S.-Japan exercises designed to strengthen deterrence and operational readiness in the region.

These activities are reinforced by significant defense recalibration in both countries. Recognizing that the regional security environment is the most complex since World War II, Japan has accelerated plans to raise defense spending to 2 percent of gross domestic product and expand capabilities in emerging domains such as unmanned systems and standoff weapons. South Korea has similarly pledged substantial increases in defense spending while continuing to expand its global defense industrial footprint, emphasizing self-defense as a foundation for peace.

Intelligence sharing has also become a central pillar of deterrence credibility. Since December 2023, the ROK, Japan and the United States have been able to share real-time data on North Korean ballistic missiles and other projectiles. This arrangement enhances early warning, improves response coordination and signals collective resolve against coercion or escalation.

Despite this progress, three challenges remain. First, long-term U.S. strategic focus cannot be taken for granted as Washington balances competing global priorities and increasing domestic pressures. Second, deep economic interdependence with China constrains the range of responses available to both Seoul and Tokyo, raising the costs of security action. Finally, Seoul and Tokyo must compartmentalize their historical and territorial disputes to advance pragmatic security cooperation.

Yet these constraints only reinforce the necessity of closer coordination. Neither South Korea nor Japan can manage the growing threats to the rules-based international order alone. The pressures they face today unite them far more than they divide them.

The recent Russian and Chinese incursions into KADIZ serve as a stark reminder that the Indo-Pacific is entering a period of heightened pressure and strategic testing. With China, Russia and North Korea pushing the boundaries of the status quo, the alignment between the South Korea and Japan is critical.

The strategic warning is clear: failure to deepen ROK-Japan cooperation will invite greater insecurity, not stability. In an era where hesitation is interpreted as weakness, sustained alignment between Seoul and Tokyo is no longer optional — it is the price of deterrence.

Retired Lt. Gen. Chun In-bum is the former commander of the Republic of Korea Army Special Warfare Command.