Since 2009, China's growing geopolitical assertiveness has triggered or exacerbated conflicts with many of its neighbors. External threat is often believed to produce domestic cohesion, such as bipartisanship or a rally-‘round-the-flag effect. However, this study uses the cases of Taiwan and South Korea to show that political parties have sometimes united around a response to Chinese pressure but at other times have been sharply divided. Despite Beijing's aggression toward Taiwan since 2016, the Kuomintang and the Democratic Progressive Party remain at odds over cross-strait policy. In contrast, South Korean conservatives and progressives united in response to Chinese economic sanctions over the deployment of Terminal High Altitude Area Defense. I argue that a country is less likely to unite against a foreign threat when a “formative rift” in its history divides political groups over national identity issues and causes them to perceive the threat differently, as in Taiwan but not South Korea. This study is based on a multilingual analysis of Taiwanese and South Korean political parties’ statements on China policy. Its findings contribute to scholarship on how international factors affect domestic politics and our understanding of China's rise.