This chapter provides a focused comparison of two key partners at the core of the US world order: Japan and the Republic of Korea (ROK). While Japan has maintained parliamentary institutions in a democratic regime to this day, the ROK experienced a transition from a personalist autocracy led by charismatic and powerful rulers to a presidential democracy. None of these two core allies ever strayed too far from US foreign policy positions. Nonetheless, the institutional settings that structured the domestic politics in Japan and the ROK made a difference in shaping their relations with the United States. In Japan, the more flexible processes of leadership change under parliamentary institutions opened up pathways for potential successors to present themselves as the solution that would restore the alignment between the foreign policy preferences of the United States and Japan respectively. In the ROK, presidential institutions created more rigid mechanisms of leadership turnover. Those arrangements allowed the ROK leaders to take positions to which the US patron grudgingly acquiesced for lack of better alternatives.