Interior Lines Will Make Land Power the Asymmetric Advantage in the Indo-Pacific

A key vulnerability in the People’s Republic of China’s anti-access/area defense makes conventional land forces—U.S. Soldiers and Marines—the joint force’s asymmetric advantage in a theater named for two oceans. The A2/AD system was built to find and destroy large, fast-moving ships and planes and to disrupt space and cyber capabilities. It was not designed to track distributed groups of mobile land forces inside its protective bubble. 

Turning AirLand Battle on its head, land forces will become essential to getting air and naval assets into the fight, especially if space and cyber are contested. In the words of former Indo-Pacific commander Adm. Harry Harris, land forces will “sink ships, neutralize satellites, shoot down missiles, and hack or jam the enemy” to punch holes in the A2/AD network. These softening attacks will provide windows for the Air Force and Navy to conduct bursts of operations, from offensive attacks to transporting forces and equipment into theater.

The key is creating interior lines—essentially, compact lines of maneuver, communications, and logistics. Interior lines provide options for military and national leaders by positioning ground forces. The Marine Corps’ Stand-In Forces concept, for example, relies on interior lines to help hold positional advantage and physically control important terrain such as maritime chokepoints. Interior lines also underwrites operational endurance – the military’s ability to fight the successive battles of a war—by positioning foundational protection, collection, command and control, and sustainment needed in conflict. 

In Europe, NATO’s interior lines are built on a backbone of posture left from the Cold War. Permanently stationed forces, stockpiles, and logistics networks that include bases, airports, and seaports are already in place and prepared to support massive military operations.

In the Indo-Pacific, however, the U.S. military lacks the interior lines it needs to fight and win. Its forces and equipment are concentrated in Korea and Japan, and the United States is perennially challenged to secure pre-conflict agreements for posture elsewhere in Asia. To oppose a PLA force with the advantages of magazine depth and its own interior lines, the U.S. military must adopt creative, practical approaches to develop the interior lines it lacks, from Southeast Asia to Australia through the first and second island chains.

Building interior lines

So the U.S. Army has reshaped the annual Defender Pacific exercise into Operation Pathways, an effort to build joint interior lines along its five foundational roles in a future high-end conflict. 

One early focus is sustainment—the cornerstone of operational endurance and an acknowledged vulnerability in future conflict. The joint force as a whole is increasing its stores of supplies and equipment west of the International Dateline through diplomatic advancements such as December’s AUSMIN agreement. Such prepositioning saves time, money, and—significant in conflict—space on ships and planes outbound from the United States. The Army is adding to these stores by creating “activity sets” in Southeast Asia during Operation Pathways. Less contentious than combat-focused prepositioned stocks, activity sets hold multi-purpose equipment and stores. In the Philippines, for example, a collapsible 6,000-person housing area is stored alongside food, water, engineering assets, airfield repair kits, and medical supplies. 

But prepositioning solves only part of the problem. In-theater sustainment requires extensive development of logistics networks, as well as extensive rehearsal, so Operation Pathways will collaborate with Air Mobility Command’s Operation Mobility Guardian in 2023. For the first time, the Army’s 8th Theater Sustainment Command will direct six continuous months of logistical and sustainment operations across the first and second island chains and Southeast Asia. Army watercraft based in Japan will transport U.S. equipment across the island chains. Forces will use equipment stored on ships for crisis since the 1990s, something the U.S. Army began practicing only last year. These efforts will culminate in July and August with complex over-the-shore logistics operations during Talisman Sabre, the largest U.S.-Australia exercise. 

Another early focus of Pathways is reshaping its military exercises toward high-end conflict, normalizing the presence and use of multi-domain capabilities and doctrine with allies and partners. In Japan, for example, last year’s bilateral Orient Shield exercise moved south to the country’s strategically significant southwestern islands, where HIMARS deployed to Amami Island for the first time and a multi-domain task force exercised with the Japanese Self-Defense Force. Months later, U.S. and Japanese two- and three-star headquarters paired up to work through Japan’s cross-domain operations and the U.S. Army’s multi-domain operations. 

 An unexpected but welcome development is that several bilateral exercises in the region have quickly become multilateral. For example, Garuda Shield, a historically army-to-army exercise with Indonesia, rapidly grew last year to a joint exercise with 4,000 troops from 14 nations.

All these exercises thicken the joint force’s interior lines organically, by moving more than 20,000 rotational troops through the region each year, expanding access to training sites in Southeast Asia, and extending exercise windows that keep U.S. forces and equipment in the region for longer periods.

Strategic implications

These tactical actions have already begun to produce strategic gains. Exercises that strengthen the ties between U.S. and regional militaries help thwart a PRC strategy for regional hegemony that depends on fracturing alliances and partnerships. The India-U.S. army exercise Yudh Abyhas 2022 shows how building interior lines helps advance regional relationships. It also gives U.S. policymakers insight into regional trends and countries’ security concerns, including shifting defense priorities and fleeting opportunities. 

Most significantly, the persistent presence and investment inherent in building interior lines signals U.S. commitment. This is critical in a region that is carefully monitoring U.S. support for Ukraine. Demonstrated commitment provides a rationale for countries to change position over time on weightier policy issues. In turn, successive diplomatic advancements like the recent 2+2 talks with Japan and Australia, unthinkable even five years ago, yield compounding opportunities as the security situation across Asia continues to sharpen. 

And should conflict come, the presence built by rotational forces building interior lines may make all the difference. A country that does not already host U.S. forces may not, for example, agree to U.S. requests to launch missiles or aircraft from within its borders. A recent CSIS report argued, for example, that the Army’s multi-domain task forces and Marine Corps’ marine littoral regiments would play no significant role in an Indo-Pacific crisis because they would be hard-pressed to gain access to nations where U.S. forces do not already sit. 

But ally and partner decision-making changes if the U.S. already has forces on the ground—say, for an exercise. A partner government may initially approve less visible or escalatory operations—collection, sustainment, command and control, or protection—by these forces. And if the country further changes its position, each U.S. capability on the ground would expand options available to military and national leaders. 

Combined with allies’ posture and capabilities and Marine Corps stand-in forces, the Army’s Operation Pathways thickens interior lines and lays the foundation for early joint operations. Individually, these efforts might appear insignificant. Together, they form a joint campaign of preparation and rehearsal that positions the U.S. military with tools to counter PLA aggression. Interior lines underwrite the U.S. military’s contributions to integrated deterrence, and in doing so, they further strengthen the network of allies and partners that bind the region’s security architecture together. 

No one knows just what it would take to win a future war in the Indo-Pacific, but one fact is clear. PLA capabilities and CCP ambitions do not afford the United States the liberty of a World War II repeat: ceding the Pacific, then spending years of blood and treasure to fight back in. Asia is a joint theater with problems that require joint solutions. Working together is the only way to win, and it begins with interior lines.

Gen. Charles Flynn has commanded U.S. Army Pacific (USARPAC) since June 2021. Previous leadership positions (including 25th Infantry Division commander, USARPAC deputy commanding general) influence his understanding of and approach to the Army's role in joint operations in USINDOPACOM.

Lt. Col. Sarah Starr is an Army Strategist at USARPAC. She has served on the Army Staff and holds degrees from the Harvard Kennedy School, University of North Georgia, and Gonzaga University.