South Korea and Japan share common challenges and liberal democratic values but have been unable to build a close security relationship, due mainly to their political differences. This paper examines the two countries’ defense cooperation in the bilateral, trilateral, and multilateral arenas over the past six years to explore how the external security environment, as well as bilateral political problems, have affected their joint activities. The paper’s analysis shows that defense cooperation between South Korea and Japan is not always a binary choice between full cooperation or no cooperation. The two countries not only weigh external threats and bilateral problems, but also ponder such factors as U.S. encouragement to adjust the scope of their cooperation. The paper also shows that while the main constraint for security cooperation between South Korea and Japan is their historical animosity, in recent years, frictions involving the countries’ armed forces have become a new reason for limiting cooperation. As a result, grassroots military-to-military exchanges between the two countries, which in the past continued despite political problems, have largely stopped since late 2018. The paper concludes by proposing that South Korea and Japan work to restore routine bilateral working-level exchanges. It also recommends that the United States continue to urge the two countries to cooperate, and that multilateral exercises be employed as forums for cooperation between the two armed forces, as that is an area that is least impacted by bilateral problems.